regulator: Assume regulators are enabled if they don't report anything

If a regulator driver does not provide a way to query if the driver is
enabled then assume that it is enabled.  This is very likely to reflect
the actual state is more useful for callers than reporting an error.

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