net: systemport: pad packets to a minimum of 68 bytes

Packets need to be at least 64 bytes to enter the switch port logic,
including the FCS, otherwise they will be discarded as RUNT packets.

With packets having Broadcom tags, the 4-bytes tag is first stripped
off the packet, and the packet length is then checked, so we need to
make sure that the packet length with FCS is at least 64 bytes.

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