media: v4l2-ctrls.h: better document the arguments for v4l2_ctrl_fill

The arguments for this function are pointers. Make it clear at
its documentation.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>