regulator: core: Add missing kerneldoc "Return" sections

kernel-doc complains about missing "Return" section for many documented
functions in the regulator core. Some with free-form return value
descriptions have been fixed in the previous patch. The remaining are
completely missing any mention of return values.

Add "Return" sections to these kerneldoc blocks with basic descriptions.
In a few cases where the functions don't call even more functions and
the error numbers are known, those are documented in detail.
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20240829085131.1361701-5-wenst@chromium.orgSigned-off-by: Mark Brown <broonie@kernel.org>