m68k: irq_node.handler() should return irqreturn_t

commit b5dc7840 ("m68k: introduce irq
controller") reverted the return type of struct irq_node.handler() from
irqreturn_t to int. Change it back to irqreturn_t, else it will give a
compiler warning when irqreturn_t is turned into an enum in the near future:

| arch/m68k/kernel/ints.c:231: warning: assignment from incompatible pointer type

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>