[BNX2]: Add 40-bit DMA workaround for 5708.

The internal PCIE-to-PCIX bridge of the 5708 has the same 40-bit DMA
limitation as some of the tg3 chips.  Set dma_mask and persistent DMA
mask to 40-bit to workaround.

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