smsc95xx: fix tx checksum offload for big endian

f7b29271

 introduced tx checksum offload support for smsc95xx,
and enabled it by default. This feature doesn't take
endianness into account, so causes most tx to fail on
those platforms.

This patch fixes the problem fully by adding the missing
conversion.

An alternate workaround is to disable TX checksum offload
on those platforms. The cpu impact of this feature is very low.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>