[BNX2]: Improve SerDes handling.

1. Add support for 2.5Gbps forced speed setting.
2. Remove a long udelay() loop and change to msleep().
3. Other misc. SerDes fixes.

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