igb: Support sending custom Ethernet FCS.

Including bad FCS, used generate frames with bad FCS
to test other system's handling of RX of bad packets.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>