[media] v4l2-ctrls.h: fix doc reference for prepare_ext_ctrls()

The prepare_ext_ctrls() function is actually internal to the
v4l2-ctrls.c implementation, so it doesn't have a declaration
for the kAPI header to reference it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/v4l2-ctrls.h

@@ -239,7 +239,7 @@ struct v4l2_ctrl {
  * @next:	Single-link list node for the hash.
  * @ctrl:	The actual control information.
  * @helper:	Pointer to helper struct. Used internally in
- *		prepare_ext_ctrls().
+ *		``prepare_ext_ctrls`` function at ``v4l2-ctrl.c``.
  *
  * Each control handler has a list of these refs. The list_head is used to
  * keep a sorted-by-control-ID list of all controls, while the next pointer