irqchip/realtek-rtl: Fix off-by-one in routing

irqchip/realtek-rtl: Fix off-by-one in routing

BugLink: https://bugs.launchpad.net/bugs/1959879
commit 91351b5dd0fd494eb2d85e1bb6aca77b067447e0 upstream.

There is an offset between routing values (1..6) and the connected MIPS
CPU interrupts (2..7), but no distinction was made between these two
values.

This issue was previously hidden during testing, because an interrupt
mapping was used where for each required interrupt another (unused)
routing was configured, with an offset of +1.

Offset the CPU IRQ numbers by -1 to retrieve the correct routing value.

Fixes: 9f3a0f34b84a ("irqchip: Add support for Realtek RTL838x/RTL839x interrupt controller")
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/177b920aa8d8610615692d0e657e509f363c85ca.1641739718.git.sander@svanheule.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>