i40iw: Remove workaround for pre-production errata

Pre-production silicon incorrectly truncates 4 bytes of the MPA
packet in UDP loopback case. Remove the workaround as it is no
longer necessary.

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>