smsc95xx: strip ethernet fcs (crc) on receive path

The smsc95xx driver was forwarding the trailing fcs on received frames
up the stack leading to confusion in tcpdump.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Tested-by: Steve Glendinning <steve.glendinning@smsc.com>
Acked-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>