Bump third_party/ot-br-posix/repo from `bd15f71` to `6112f98` #1677

Job	Run time
Unit / Integration Tests (clang)	20m 12s
Unit / Integration Tests (rotating_device_id)	18m 4s
Unit / Integration Tests (mbedtls)	14m 55s
Unit / Integration Tests (main)	13m 51s
Unit / Integration Tests (icd)	43m 46s
	1h 50m 48s

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