i40iw: Remove unnecessary check for moving CQ head

In i40iw_cq_poll_completion, we always move the tail. So there is
no reason to check for overflow everytime we move the head.

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