ahci: Do not receive interrupts sent by dummy ports

In multiple MSI mode all AHCI ports (including dummy) get assigned
separate MSI vectors and (as result of execution
pci_enable_msi_exact() function) separate IRQ numbers, (mapped to the
MSI vectors).

Therefore, although interrupts from dummy ports are not desired they
are still enabled. We do not request IRQs for dummy ports, but that
only means we do not assign AHCI-specific ISRs to corresponding IRQ
numbers.

As result, dummy port interrupts still could come and traverse all the
way from the PCI device to the kernel, causing unnecessary overhead.

This update disables IRQs for dummy ports and prevents the described
issue.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Tested-by: David Milburn <dmilburn@redhat.com>
Cc: linux-ide@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: 5ca72c4f ("AHCI: Support multiple MSIs")