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>

arch/m68k/include/asm/irq_mm.h

@@ -3,6 +3,7 @@
 
 #include <linux/linkage.h>
 #include <linux/hardirq.h>
+#include <linux/irqreturn.h>
 #include <linux/spinlock_types.h>
 
 /*
@@ -80,7 +81,7 @@ struct pt_regs;
  * interrupt source (if it supports chaining).
  */
 typedef struct irq_node {
-	int		(*handler)(int, void *);
+	irqreturn_t	(*handler)(int, void *);
 	void		*dev_id;
 	struct irq_node *next;
 	unsigned long	flags;