genirq: Annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough. The
fallthrough in __irq_set_trigger() lacks an annotation. Add it.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190114203154.17125-1-malat@debian.org