media: ar0521: fix Kconfig: VIDEO_V4L2 -> VIDEO_DEV

VIDEO_V4L2 no longer exists, instead use VIDEO_DEV. Without this change
this driver cannot be selected.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>