enic: enable ethtool LRO support

Enable ethtool support for get/set_flags so LRO can be turned on/off
by fwding drivers such as the bridge driver.  LRO is not compatible
with fwding drivers.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>