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.be


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>