i40iw: Fix QP flush to not hang on empty queues or failure

When flush QP and there are no pending work requests, signal completion
to unblock i40iw_drain_sq and i40iw_drain_rq which are waiting on
completion for iwqp->sq_drained and iwqp->sq_drained respectively.
Also, signal completion if flush QP fails to prevent the drain SQ or RQ
from being blocked indefintely.

Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>