powerpc/fsl_msi: fix support for multiple MSI ranges

Commit 6820fead ("powerpc/fsl_msi: Handle msi-available-ranges better") added
support for multiple ranges in the msi-available-ranges property, but it
miscalculated the MSIR index when multiple ranges are used.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>