[POWERPC] powermac i2c: Use mutex

Convert the semaphores in low_i2c that are used as mutexes to real
mutexes.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>