tg3: Fix jumbo loopback test on 5719

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: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>