[NETDRV]: Perform missing csum_offset conversions

When csum_offset was introduced we did a conversion from csum to
csum_offset where applicable.  A couple of drivers were missed in
this process.

It was harmless to begin with since the two fields coincided.  Now
that we've made them different with the addition of csum_start, the
missed drivers must be converted or they can't send packets out at
all that require checksum offload.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>