regulator: If we can't configure optimum mode we're always in the best one

regulator: If we can't configure optimum mode we're always in the best one

If either a regulator driver can't tell us what the optimum mode is (or
doesn't have modes in the first place) or the system doesn't allow DRMS
changes then it's more helpful for users to just say that we're in the
optimal mode, even if it's from a selection of one.

Still report errors if the process of picking and setting a mode changes as
this may indicate that we're stuck in a low power mode and unable to deliver
a higher current that the consumer just asked for.

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