MIPS: SB1250: Restore dropped irq_mask function

Commit d6d5d5c4 (MIPS: Sibyte: Convert to new irq_chip functions)
removed the mask function which breaks irq_shutdown(). Restore it.
Reported-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2460/Tested-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/sibyte/sb1250/irq.c

@@ -123,6 +123,13 @@ static int sb1250_set_affinity(struct irq_data *d, const struct cpumask *mask,
 }
 #endif
 
+static void disable_sb1250_irq(struct irq_data *d)
+{
+	unsigned int irq = d->irq;
+
+	sb1250_mask_irq(sb1250_irq_owner[irq], irq);
+}
+
 static void enable_sb1250_irq(struct irq_data *d)
 {
 	unsigned int irq = d->irq;
@@ -180,6 +187,7 @@ static struct irq_chip sb1250_irq_type = {
 	.name = "SB1250-IMR",
 	.irq_mask_ack = ack_sb1250_irq,
 	.irq_unmask = enable_sb1250_irq,
+	.irq_mask = disable_sb1250_irq,
 #ifdef CONFIG_SMP
 	.irq_set_affinity = sb1250_set_affinity
 #endif