mv643xx_eth: deal with unexpected ethernet header sizes

When the IP header doesn't start 14, 18, 22 or 26 bytes into the packet
(which are the only four cases that the hardware can deal with if asked
to do IP checksumming on transmit), invoke the software checksum helper
instead of letting the packet go out with a corrupt checksum inserted
into the packet in the wrong place.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>