libbladeRF: Don't hold buffer lock while submitting a TX buffer

The stream callbacks hold a stream lock when executed. This
stream lock is obtained in bladerf_submit_stream_buffer() calls.

If the sync_tx call does not release its buffer lock before submitting a
buffer via bladerf_submit_stream_buffer(), deadlock can occur:
    - sync_tx is called and we're ready to submit data
        + the sync buffer lock is acquired
        + In bladerf_submit_stream_buffer() the stream lock is acquired
    - libusb callback fires, and the stream lock is acquired
        + The user (sync tx) callback is then called, which acquires the
        buffer lock
    - Deadlock