staging: wfx: introduce a counter of pending frames

This counter will be useful to know which queue is least full in a
further patch.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-22-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>