smsc9420: fix big endian rx checksum offload

The cpu_to_le16 here looks suspicious to me, I don't think we need it
because put_unaligned_le16 also does this.

I don't currently have any big endian hardware with a PCI bus available
to test on, so I haven't been able to verify this.

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