regulator: Fix some kerneldoc rendering issues

There are some minor textual changes in here as well, mostly to enable()
and disable() but the primary goal of these changes is to fix
misrenderings of the kerneldoc documentation for the regulator API.

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