ahci: drop intx manipulation on msi enable

There's no need to turn off intx explicitly on msi enable.  This is
automatically handled by pci.  Drop it.

This might be needed on machines if the BIOS turns intx off during
boot.  However, there's no evidence of such behavior for ahci and
the only such case seems to be ICH5 PATA according to ata_piix.
Also, given the way ahci operates, it's highly unlikely BIOS ever
disables IRQ for the controller.  However, as this change has slight
possibility of introducing failure, please schedule it for #upstream.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>