media: v4l2-ctrl: Add new helper v4l2_ctrl_ptr_create

This helper function simplifies the code by not needing a union
v4l2_ctrl_ptr and an assignment every time we need to use
a ctrl_ptr.

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>