i40iw: Convert page_size to encoded value

Passed in page_size was used as encoded value for writing
the WQE and passed in value was usually 4096. This was
working out since bit 0 was 0 and implies 4KB pages,
but would not work for other page sizes.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Henry Orosco <henry.orosco@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>