libbladeRF: Moved byte <-> SC16Q11 conversion to bladerf_priv.h

This is part of the cleanup for issue #190.
libbladeRF: Moved system gain functions, deadlock fix

Moved system gain functions from bladerf_priv.c to gain.c as part of
the cleanup efforts for issue #190.

In doing so, a stray MUTEX_LOCK() (without a matching MUTEX_UNLOCK) was
removed, which caused deadlocks.