MIPS: SB1250: Restore dropped irq_mask function

Commit d6d5d5c4a (MIPS: Sibyte: Convert to new irq_chip functions)
removed the mask function which breaks irq_shutdown(). Restore it.

Reported-by: Matt Turner <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/2460/
Tested-by: Matt Turner <[e-mail]>
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/sibyte/sb1250/irq.c

···
 }
 #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;
···
 	.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