regulator: rohm-regulator: linear voltage support

regulator: rohm-regulator: linear voltage support

The helper for obtaining HW-state based DVS voltage levels currently only
works for regulators using linear-ranges. Extend support to regulators with
simple linear mappings and add also proper error path if pickable-ranges
regulators call this.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>