dp8393x: Implement packet size limit and RBAE interrupt

dp8393x: Implement packet size limit and RBAE interrupt

Add a bounds check to prevent a large packet from causing a buffer
overflow. This is defensive programming -- I haven't actually tried
sending an oversized packet or a jumbo ethernet frame.

The SONIC handles packets that are too big for the buffer by raising
the RBAE interrupt and dropping them. Linux uses that interrupt to
count dropped packets.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit ada74315270d1dcabf4c9d4fece19df7ef5b9577)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>