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>