uio_pdrv_genirq: get irq through platform resource if not set otherwise

Platform devices are configured through platform resources. The interrupt
in the driver uio_pdrv_genirq is instead configured through a side channel
i.e. the platform data structure. Make it possible to use the generic
configuration scheme via platform resource.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: "Hans J. Koch" <hjk@hansjkoch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>