genirq: Correctly annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough. The
fallthrough in __handle_irq_event_percpu() has a fallthrough annotation
which is followed by an additional comment and is not recognized by GCC.

Separate the 'fall through' and the rest of the comment with a dash so the
regular expression used by GCC matches.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190114203633.18557-1-malat@debian.org