cavium: Return negative value when pci_alloc_irq_vectors() fails

During the process of driver probing, the probe function should return < 0
for failure, otherwise, the kernel will treat value > 0 as success.

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>