[media] cobalt: use pci_irq_allocate_vectors()

Simplify the interrupt setup by using the new PCI layer helpers.

Despite using pci_enable_msi_range(), this driver was only requesting a
single MSI vector anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>