m68k/sun3: Get interrupts working again

SUN3 Intregister is not for enabling individual interrupts, but to enable
special interrupts. So using it for interrupt enable/disable was wrong.
The clock interrupt needs some special treatment to keep ticking.

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