i40iw: Fix incorrect assignment of SQ head

The SQ head is incorrectly incremented when the number
of WQEs required is greater than the number available.
The fix is to use the I40IW_RING_MOV_HEAD_BY_COUNT
macro. This checks for the SQ full condition first and
only if SQ has room for the request, then we move the
head appropriately.

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