wil6210: do not stop Tx queue on packet drop

Packet drop may be caused by various flows, like disconnect
while Tx packets was queued; this should not lead to stopping
of the Tx queue, or all Tx get stalled.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>