libbladeRF: Clear sync buffer "in-flight" status when restarting stream

When restarting a stream, we assume in-flight buffers are lost. These
must be marked empty so they can be re-used.

Also in this commit:
    - Cleaned up unnecessary unlock, relock, unlock
    - Fixed return value check of pthread_cond_timedwait
libbladeRF: [WIP] Renamed and moved backend function table items
libbladeRF: [WIP] Started copying over USB fns