[diag] Move otPlatDiagModeGet() check to ProcessCmd

All diagnostic commands (instead of `diag start`) should fail if
device is not in diagnostic mode. Previously it was verified by
each command's process method (with a missing check in
`ProcessEcho` and `ProcessGpio`). This commit moves the check
directly to `ProcessCmd` and cleans up redundant code.

Additionally clean the documentation and align the code as for
some commands `status 0x00` was added on success and for some not.

Signed-off-by: Maciej Baczmanski <maciej.baczmanski@nordicsemi.no>

Author: maciejbaczmanski

src/core/diags/README.md

@@ -38,7 +38,7 @@ Start diagnostics mode.
 ```bash
 > diag start
 start diagnostics mode
-status 0x00
+Done
 ```
 
 ### diag channel
@@ -48,6 +48,7 @@ Get the IEEE 802.15.4 Channel value for diagnostics module.
 ```bash
 > diag channel
 channel: 11
+Done
 ```
 
 ### diag channel \<channel\>
@@ -57,7 +58,7 @@ Set the IEEE 802.15.4 Channel value for diagnostics module.
 ```bash
 > diag channel 11
 set channel to 11
-status 0x00
+Done
 ```
 
 ### diag cw start
@@ -123,6 +124,7 @@ Get the tx power value(dBm) for diagnostics module.
 ```bash
 > diag power
 tx power: -10 dBm
+Done
 ```
 
 ### diag power \<power\>
@@ -132,7 +134,7 @@ Set the tx power value(dBm) for diagnostics module.
 ```bash
 > diag power -10
 set tx power to -10 dBm
-status 0x00
+Done
 ```
 
 ### diag powersettings
@@ -173,7 +175,7 @@ Send the frame set by `diag frame` if length is omitted. Otherwise overwrite the
 ```bash
 > diag send 20 100
 sending 0x14 packet(s), length 0x64
-status 0x00
+Done
 ```
 
 ### diag repeat \<delay\> [length]
@@ -185,7 +187,7 @@ Send the frame set by `diag frame` if length is omitted. Otherwise overwrite the
 ```bash
 > diag repeat 100 100
 sending packets of length 0x64 at the delay of 0x64 ms
-status 0x00
+Done
 ```
 
 ### diag repeat stop
@@ -195,7 +197,7 @@ Stop repeated packet transmission.
 ```bash
 > diag repeat stop
 repeated packet transmission is stopped
-status 0x00
+Done
 ```
 
 ### diag radio sleep
@@ -205,7 +207,7 @@ Enter radio sleep mode.
 ```bash
 > diag radio sleep
 set radio from receive to sleep
-status 0x00
+Done
 ```
 
 ### diag radio receive
@@ -215,7 +217,7 @@ Set radio from sleep mode to receive mode.
 ```bash
 > diag radio receive
 set radio from sleep to receive on channel 11
-status 0x00
+Done
 ```
 
 ### diag radio receive \[async\] \<number\> \[lpr\]
@@ -245,6 +247,7 @@ Return the state of the radio.
 ```bash
 > diag radio state
 sleep
+Done
 ```
 
 ### diag radio enable
@@ -312,6 +315,7 @@ received packets: 10
 sent packets: 10
 first received packet: rssi=-65, lqi=101
 last received packet: rssi=-64, lqi=98
+Done
 ```
 
 ### diag stats clear
@@ -321,6 +325,7 @@ Clear statistics during diagnostics mode.
 ```bash
 > diag stats clear
 stats cleared
+Done
 ```
 
 ### diag gpio get \<gpio\>
@@ -384,7 +389,7 @@ first received packet: rssi=-65, lqi=101
 last received packet: rssi=-61, lqi=98
 
 stop diagnostics mode
-status 0x00
+Done
 ```
 
 ### diag rcp