ixgbe: enable SCTP checksum offload for X540

X540 supports SCTP checksum offload so enable it.  It was overlooked when X540
support was initially added to the driver.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Evan Swanson <evan.swanson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>