uio: Request/free irq separate from dev lifecycle

Separate irq request/free from the device lifecycle.
After device unregister the parent module can call pci_disable_msi.
>From the PCI MSI how to:

"Before calling this function, a device driver must always call free_irq()
on any interrupt for which it previously called request_irq().
Failure to do so results in a BUG_ON(), leaving the device with
MSI enabled and thus leaking its vector."

So we need to separately free the irq at unregister to allow the device
to be kept around in the case of it still having open FDs.

Signed-off-by: Brian Russell <brussell@brocade.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>