regulator: fix header file missing kernel-doc

Add regulator header file missing kernel-doc:

Warning(include/linux/regulator/driver.h:117): No description found for parameter 'set_mode'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
cc:	Liam Girdwood <lrg@slimlogic.co.uk>
cc:	Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>

include/linux/regulator/driver.h

@@ -50,6 +50,7 @@ enum regulator_status {
  * @set_current_limit: Configure a limit for a current-limited regulator.
  * @get_current_limit: Get the configured limit for a current-limited regulator.
  *
+ * @set_mode: Set the configured operating mode for the regulator.
  * @get_mode: Get the configured operating mode for the regulator.
  * @get_status: Return actual (not as-configured) status of regulator, as a
  *	REGULATOR_STATUS value (or negative errno)