irq: add remove_irq() for freeing of setup_irq() irqs

Impact: add new API

This patch adds a remove_irq() function for releasing
interrupts requested with setup_irq().

Without this patch we have no way of releasing such
interrupts since free_irq() today tries to kfree()
the irqaction passed with setup_irq().

Signed-off-by: Magnus Damm <damm@igel.co.jp>
LKML-Reference: <20090312120542.2926.56609.sendpatchset@rx1.opensource.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>