regulator: core: Fast path non-deferred disables

Users (especially framework code) may end up passing in a zero deferral
time depending on runtime conditions or configuration. If they do then
just call regulator_disable() directly to save scheduling.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>