ahci: always fall back to single-MSI mode

Don't try to guess what the errors from pci_irq_alloc_vectors mean, as
that's too fragile.  Instead always try allocating a single vector
when multi-MSI mode fails.  This makes various intel Desktop and
Laptop CPUs use MSI again.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Michael Marley <michael@michaelmarley.com>
Tested-by: Michael Marley <michael@michaelmarley.com>
Fixes: 0b9e2988 ("ahci: use pci_alloc_irq_vectors")
Signed-off-by: Tejun Heo <tj@kernel.org>