spidernet: checksum and ethtool

It doesn't look like spidernet hardware can really checksum all protocols,
the code looks like it does IPV4 only.  If so, it should use NETIF_F_IP_CSUM
instead of NETIF_F_HW_CSUM.

The driver doesn't need it's own get/set for ethtool tx csum, and it
should use the standard ethtool_op_get_link.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>