of: dma: Move range size workaround to of_dma_get_range()

Invalid dma-ranges values should be worked around when retrieving the
DMA range in of_dma_get_range(), not by all callers of the function.
This isn't much of a problem now that we have a single caller, but that
situation will change when moving DMA configuration to device probe
time.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>