e1000: Support RX-FCS flag.

This allows the NIC to pass the Ethernet Frame Checksum
(FCS) up the stack.  Useful when sniffing packets.

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>