skge: fix checksum byte order

Running old skge driver on PowerPC causes checksum errors
because hardware reported 1's complement checksum is in little-endian
byte order.

Reported-by: Benoit <benoit.sansoni@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>