[media] v4l2-ctrl.h: fix comments

The comments for the unlocked v4l2_ctrl_s_ctrl* functions were wrong (copy
and pasted from the locked variants). Fix this, since it is confusing.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>