m68k: Correct number of interrupts for Sun3

Only attempt to initialize the amount of interrupts a sun3 actually has...

Signed-off-by: Sam Creasey <sammy@sammy.net>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>