ahci: always clear all bits in irq_stat

Some AHCI controllers (ICH7 was reported) set pending bit in
HOST_IRQ_STAT for non-existent ports and when it's not cleared falls
into IRQ storm.  Always clear full irq_stat instead of only the bits
that are handled.  As nothing changes for recognized ports, the risk
of breaking things is pretty low.

Reported and verified by Philipp Thomas in the following suse
bugzilla.

https://bugzilla.novell.com/attachment.cgi?id=215692

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Philipp Thomas <pth@novell.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>