hso: IP checksuming doesn't work on GE0301 option cards

There is definitly a problem, that some option cards send up broken
IP pakets leading to corrupted IP packets. These corruptions aren't
detected, because the driver claims that the packets are already
checksummed. This change removes the CHECKSUM_UNNECESSARY option
and let IP detect broken data.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: David S. Miller <davem@davemloft.net>