staging: wfx: fix counter overflow

Some weird behaviors were observed when connection is really good and
packets are small. It appears that sometime, number of packets in queues
can exceed 255 and generate an overflow in field usage_count.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-4-Jerome.Pouiller@silabs.com


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