net: systemport: Add a check for oversized packets

Occasionnaly we may get oversized packets from the hardware which exceed
the nomimal 2KiB buffer size we allocate SKBs with. Add an early check
which drops the packet to avoid invoking skb_over_panic() and move on to
processing the next packet.

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