m68k/irq: Vector ints need a valid interrupt handler

To get vectored interrupts working we need to switch from the default
handler handle_bad_irq() to something more sensible. Tested on a MVME177
board.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>