Bump third_party/ot-br-posix/repo from `bd15f71` to `b9210e7` #296

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 57m 40s
Build on Linux (gcc_debug)	1h 3m 22s
Build on Linux (python_lib)	29m 49s
Build on Darwin (clang, python_lib, simulated)	1h 41m 20s
Build on Linux (coverage)	14m 2s
Build on Linux (python lighting-app)	11m 55s
	5h 38m 8s

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