Clear spurious irq stat information when adding irq handler

Any newly added irq handler may obviously make any old spurious irq
status invalid, since the new handler may well be the thing that is
supposed to handle any interrupts that came in.

So just clear the statistics when adding handlers.

Pointed-out-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>