[SC92031] Using padto turned driver into an IPv6-only interface

IPv4 would work with this driver only with static arp table entries,
the patch  reverts a padto introduced in

 commit 26a17b7b


 sc92031: start transmit return value bugfix

The padto does not work because the driver code evaluates `len' later on and
there are cases where skb->len is not updated accordingly.

This was observed with ARP frames (skb->len = 42 bytes, !skb_cloned(),
skb_tailroom = 84 bytes). Then in skb_pad(), the first condition is true, where
skb->len is not updated. As a consequence, the driver uses 42 bytes instead of
the 60 bytes, and the ARP frame never makes it onto the wire.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>