uio/uio_pci_generic: don't fail probe if pdev->irq == NULL

Some userspace drivers and frameworks only poll and do not
require interrupts to be available and enabled on the
PCI device.  So remove the requirement that an IRQ is
assigned.  If an IRQ is not assigned and a userspace
driver tries to read()/write(), the generic uio
framework will just return -EIO.

This allows binding uio_pci_generic to devices which
cannot get an IRQ assigned, such as an NVMe controller
behind Intel Volume Management Device (VMD), since VMD
does not support INTx interrupts.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>