Lots of regulator drivers have checks in their map_voltage() functions
to verify that the result of the mapping is in the range originally
specified. Factor these out in the core and provide a bit of extra
defensiveness for other drivers by doing the check in the core.

Since we're now doing a list_voltage() earlier move the current mapping
back to a voltage out into the set_voltage() call to save redoing it.
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>