ahci: only try to use multi-MSI mode if there is more than 1 port

We should only try to allocate multiple MSI or MSI-X vectors if the device
actually has multiple ports.  Otherwise pci_alloc_irq_vectors will return
a single vector due to n_ports = 1, in which case we shouldn't set the
AHCI_HFLAG_MULTI_MSI flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: 0b9e2988 ("ahci: use pci_alloc_irq_vectors")
Reported-by: Emmanuel Benisty <benisty.e@gmail.com>
Tested-by: Emmanuel Benisty <benisty.e@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>