sh: mach-highlander: Handle virq offset in cascaded IRL demux

sh: mach-highlander: Handle virq offset in cascaded IRL demux

Take into account the virq offset when translating cascaded IRL
interrupts.

Fixes: a8ac2961148e8c72 ("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/4fcb0d08a2b372431c41e04312742dc9e41e1be4.1688908186.git.geert+renesas@glider.be
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>