regulator: Bootstrap wm831x DVS VSEL value from ON VSEL if not already set

If we don't have a DVS VSEL value already set when we start up then start
it off with the value currently being used for ON.

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