[PATCH] via-velocity: use NETIF_F_IP_CSUM (hardware only support IPv4)

At least some versions of the via-velocity hardware only support
checksumming IPv4 frames in hardware.  However, the driver is currently
setting the NETIF_F_HW_CSUM flag, which indicates support for more than
just IPv4.  This results in errors when trying to use IPv6 over
via-velocity hardware.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>