cxgbe(4): Add NIC_ETHOFLD to the NIC capabilities allowed by the driver
by default.

This is the first of a series of commits that will add support for
RATELIMIT kernel option to the base if_cxgbe driver, for use with
ordinary NIC traffic "flows". RATELIMIT is already supported by t4_tom
for the fully-offloaded TCP connections that it handles.

Sponsored by: Chelsio Communications