staging: wfx: fix potential deadlock in wfx_tx_flush()

staging: wfx: fix potential deadlock in wfx_tx_flush()

wfx_tx_flush() wait there are no more frame in device buffer. However,
this event may never happens since wfx_tx_flush() don't forbid to
enqueue new frames.

Note that wfx_tx_flush() should only ensure that all frames currently in
hardware queues are sent. So the current code is more restrictive that
it should.

Note that wfx_tx_flush() release the lock before to return while
wfx_tx_lock_flush() keep the lock.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-31-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>