net: ethoc: Correctly pad short packets

Even though the hardware can be doing zero padding, we want the SKB to
be going out on the wire with the appropriate size. This fixes packet
truncations observed with e.g: ARP packets.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>