sh: mach-r2d: Handle virq offset in cascaded IRL demux

sh: mach-r2d: Handle virq offset in cascaded IRL demux

When booting rts7751r2dplus_defconfig on QEMU, the system hangs due to
an interrupt storm on IRQ 20.  IRQ 20 aka event 0x280 is a cascaded IRL
interrupt, which maps to IRQ_VOYAGER, the interrupt used by the Silicon
Motion SM501 multimedia companion chip.  As rts7751r2d_irq_demux() does
not take into account the new virq offset, the interrupt is no longer
translated, leading to an unhandled interrupt.

Fix this by taking into account the virq offset when translating
cascaded IRL interrupts.

Fixes: a8ac2961148e8c72 ("sh: Avoid using IRQ0 on SH3 and SH4")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/r/fbfea3ad-d327-4ad5-ac9c-648c7ca3fe1f@roeck-us.net
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/2c99d5df41c40691f6c407b7b6a040d406bc81ac.1688901306.git.geert+renesas@glider.be
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>