sh: mach-dreamcast: Handle virq offset in cascaded IRQ demux

Take into account the virq offset when translating cascaded interrupts.

Fixes: a8ac2961 ("sh: Avoid using IRQ0 on SH3 and SH4")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/7d0cb246c9f1cd24bb1f637ec5cb67e799a4c3b8.1688908227.git.geert+renesas@glider.beSigned-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

arch/sh/boards/mach-dreamcast/irq.c

@@ -108,13 +108,13 @@ int systemasic_irq_demux(int irq)
 	__u32 j, bit;
 
 	switch (irq) {
-	case 13:
+	case 13 + 16:
 		level = 0;
 		break;
-	case 11:
+	case 11 + 16:
 		level = 1;
 		break;
-	case  9:
+	case 9 + 16:
 		level = 2;
 		break;
 	default: