Loopback on 9K packet fails because the chip has a DMA limit of 4K.  The
loopback test logic uses a single BD for simplicity.  Fix it by reducing
the jumbo packet size to the DMA limit.
Signed-off-by: NMichael Chan <mchan@broadcom.com>
Reviewed-by: NBenjamin Li <benli@broadcom.com>
Reviewed-by: NMatt Carlson <mcarlson@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>