wfx_tx_queue_get_num_queued() can take advantage of BIT() instead of
maintaining one variable for a counter and another for a mask.

In add, wfx_tx_queue_get_num_queued() has no real reason to return a
size_t instead of an int.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-64-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>