tg3: Break larger frags into 4k chunks for 5719

The 5719 has bug where RDMAs larger than 4k can cause problems.  This
patch works around the problem by dividing larger DMA requests into
something the hardware can handle.

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