[NET] snap: needs hardware checksum fix

The SNAP code pops off it's 5 byte header, but doesn't adjust
the checksum. This would cause problems when using device that
does IP over SNAP and hardware receive checksums.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>