em: skip rxcsum offload processing when disabled

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

Reviewed by: gallatin, kbowling, erj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33155

(cherry picked from commit d0633af7658fe02bc1af8639ef6dbb0d029c1b65)