media: i2c: isl7998x: make const array isl7998x_video_in_chan_map static

Don't populate the read-only array isl7998x_video_in_chan_map on the
stack but instead make it static. Also makes the object code a little
smaller.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>