[BNX2]: Put MII register offsets in the bnx2 struct.

The 5709 Serdes device uses non-standard MII register offsets.  This
re-structuring will make it easier to support 5709 Serdes.

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