In c4iw_drain_sq/rq(), if the particular queue is already empty
then don't block.

Fixes: ce4af14d94aa ('iw_cxgb4: add queue drain functions')
Signed-off-by: NSteve Wise <swise@opengridcomputing.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>