e100: Support RXFCS feature flag.

This allows e100 to be configured to append the
Ethernet FCS to the skb.

Useful for sniffing networks.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>