m68k: Switch to asm-generic/hardirq.h

Classic m68k with MMU was converted to generic hardirqs a long time ago,
and there are no longer include dependency issues preventing the direct
use of asm-generic/hardirq.h.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200112174854.2726-1-geert@linux-m68k.org