em: skip rxcsum offload processing when disabled

[vmaffione]

Summary

Similarly to the other Intel drivers, don't try to process
RX checksum offloads when this feature (IFCAP_RXCSUM) is
disabled.

[gallatin]

I don't like that the drivers are dealing in ifps and calling things like if_getcapenable(). Establishing an interface that allowed drivers to have no knowledge of these details was one of the motivations for iflib.

The alternative would be to have iflib itself tell the driver if rxcsum offload was desired by setting a flag in the if_rxd_info_t

Also, if we stick with this approach, the ifp is passed into the driver via if_rxd_info_t's iri_ifp, so the iflib_get_ifp() call is not needed.

[vmaffione]

Use ri->iri_ifp

[vmaffione]

Thanks. I think we may get there (getting rid of ifp) at a later step, since some reworking would actually needed within iflib.
This change is meant to simply align em(4) to the other drivers.

[kbowling]

LGTM