media: v4l2-ctrls-api.c: add back dropped ctrl->is_new = 1

The patch adding support for dynamically allocated arrays accidentally
dropped the line setting ctrl->is_new to 1, thus new string values were
always ignored.

Fixes: fb582cba ("media: v4l2-ctrls: add support for dynamically allocated arrays.")
Reported-by: Alice Yuan <alice.yuan@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>