m68k/irq: Rename setup_irq() to m68k_setup_irq() and make it static

It has nothing to do with the standard one in <linux/irq.h>

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>