Bump third_party/ot-br-posix/repo from `bd15f71` to `6112f98` #1488

Job	Run time
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 27m 52s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	5m 22s
REPL Tests - Linux	5m 30s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 58m 46s
REPL Tests - Darwin	0s
	3h 37m 30s

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