lib: add linear range get selector within

Add linear range get selector within for choose closest selector
between minimum and maximum selector.

Signed-off-by: Gene Chen <gene_chen@richtek.com>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>