regulator: fix regulator/consumer.h kernel-doc warning

Fix kernel-doc warning about internal/private data by marking it
as "private:" so that kernel-doc will ignore it.

Warning(include/linux/regulator/consumer.h:128): No description found for parameter 'ret'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>