media: radio: si4713: constify static struct v4l2_ioctl_ops

The only usage of radio_si4713_ioctl_ops is to assign its address to the
ioctl_ops field in the video_device struct, which is a pointer to const.
Make it const to allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>