PNP: set IRQ index in sysfs "set irq" interface

We have to set the ISAPNP register index when setting an IRQ via the sysfs
interface.  We already do it for IO, MEM, and DMA resources; I just missed the
IRQ one.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>