[PATCH] ahci: remove IRQ mask clearing from init_controller()

Initial IRQ mask clearing is done by libata-core by freezing all ports
prior to requesting IRQ.  Remove redundant IRQ clearing from
init_controller().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>