regulator: Only apply voltage constraints from consumers that set them

When applying the set_voltage() requests from consumers skip over those
consumers that haven't set anything, otherwise we'll come out with a
maximum voltage of zero.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>