bnx2: Enable auto-mdix when autoneg is disabled.

Auto-mdix currently only works if autoneg is enabled.  This patch enables
auto-mdix all the time by setting a bit in a PHY register.  Define
meaningful constants for this PHY registers.

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