scsi: vmw_pvscsi: handle the return value from pci_alloc_irq_vectors correctly

It returns the number of vectors allocated when successful, so check for
a negative error only.

Fixes: 2e48e349 ("scsi: vmw_pvscsi: switch to pci_alloc_irq_vectors")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Loïc Yhuel <loic.yhuel@gmail.com>
Tested-by: Loïc Yhuel <loic.yhuel@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>