i40evf: Be much more verbose about what we can and cannot offload

This change makes it so that we are much more robust about defining what we
can and cannot offload.  Previously we were performing no checks.  This
should bring us up to parity with the i40e PF driver.

In addition the device only supports GSO as long as the MSS is 64 or
greater.  We were not checking this so an MSS less than that was resulting
in Tx hangs.

Change-ID: If533553ec92fc6ba694eab6ac81fdaf3004f3592
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>