S2io: Enable msi-x link interrupts.

- Enable msi-x link interrupts because the timer based scheduler was getting
cancelled causing the link state to be lost with repetitive card up/downs
when changing the mtu.
- Unmask mac_rmac_link interrupts only for Xframe I and prevent a spurious
link interrupt in Xframe II.
- Stop the tx queue and indicate link down when card is down

Signed-off-by: Santosh Rastapur <santosh.rastapur@neterion.com>
Signed-off-by: Ramkrishna Vepa <ram.vepa@neterion.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>