net: systemport: Remove short packet padding

Short packet padding added to the driver is only necessary when using
Broadcom tags, but since this is now taken care of net/dsa/tag_brcm.c,
we are guaranteed being given correctly padded packets.

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