of/irq: Fix msi-map calculation for nonzero rid-base

The existing msi-map code is fine for shifting the entire RID space
upwards, but attempting finer-grained remapping reveals a bug. It turns
out that we are mistakenly treating the msi-base part as an offset, not
as a new base to remap onto, so things get squiffy when rid-base is
nonzero. Fix this, and at the same time add a sanity check against
having msi-map-mask clash with a nonzero rid-base, as that's another
thing one can easily get wrong.

CC: <stable@vger.kernel.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Tested-by: Stuart Yoder <stuart.yoder@nxp.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: Rob Herring <robh@kernel.org>