[TG3]: Add rx BD workaround

Add workaround to limit the burst size of rx BDs being DMA'ed to the
chip.  This works around hardware errata on a number of 5750, 5752,
and 5755 chips.

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