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

Fixes: 3bb434cd ("vmw_vmci: switch to pci_irq_alloc_vectors")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Loïc Yhuel <loic.yhuel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>