C6X/megamode-pic: Prepare megamod_irq_cascade for irq argument removal

The irq argument of most interrupt flow handlers is unused or merily
used instead of a local variable. The handlers which need the irq
argument can retrieve the irq number from the irq descriptor.

Search and update was done with coccinelle and the invaluable help of
Julia Lawall.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Mark Salter <msalter@redhat.com>
Cc: linux-c6x-dev@linux-c6x.org