[PPC] minor irq handler cleanups

- whitespace cleanups

- remove pointless prototype (uses always follow func implementation)

- 'irq' argument is used here purely as a local variable.  rename
  argument to 'dummy' and define 'irq' as local to make this plain.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>