regulator: ab8500-ext: Provide a set_voltage call-back operation

When registering regulators which have a single voltage through Device
Tree, the framework insists that the specified voltage is actually set.
Well in order to do that we need to provide this call-back, where we
check that the value is sane and return without error. Not that the
selector isn't populated, but in our case list_voltage doesn't actually
use it, so we're good.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>