scsi: mpt3sas: switch to pci_alloc_irq_vectors

Cleanup the MSI-X handling allowing us to use the PCI-layer provided
vector allocation.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>