UIO: uio_pdrv_genirq: allow custom irq_flags

I can't think of a reason why the driver prevents people from setting any
custom bits in their platform device, but I can think of some reasons for
allowing custom flags.  Like setting the IRQF_TRIGGER_... bits.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Hans J. Koch <hjk@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>