[low-power] enhance `mCslFrameRequestAheadUs` calculation for RCP #93

Job	Run time
backbone-router	14m 27s
backbone-router	14m 27s
BR (mDNSResponder, TREL=0)	32m 19s
BR (mDNSResponder, TREL=0)	32m 19s
BR (mDNSResponder, TREL=1)	31m 52s
BR (mDNSResponder, TREL=1)	31m 52s
BR MATN (mDNSResponder, TREL=0)	19m 39s
BR MATN (mDNSResponder, TREL=0)	20m 4s
BR LowPower (mDNSResponder, TREL=0)	9m 7s
BR LowPower (mDNSResponder, TREL=0)	9m 7s
BR nat64 (mDNSResponder, TREL=0)	25m 35s
BR nat64 (mDNSResponder, TREL=0)	25m 35s
BR (avahi, TREL=0)	32m 33s
BR (avahi, TREL=0)	32m 33s
BR (avahi, TREL=1)	31m 1s
BR (avahi, TREL=1)	31m 1s
BR core-firewall (avahi, TREL=0)	31m 41s
BR core-firewall (avahi, TREL=0)	31m 41s
upload-coverage	0s
upload-coverage	1m 5s
	7h 37m 58s

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