media: vivid: fix incorrect capabilities for radio

The vivid driver has two custom controls that change the behavior of RDS.
Depending on the control setting the V4L2_CAP_READWRITE capability is toggled.
However, after an earlier commit the capability was no longer set correctly.
This is now fixed.

Fixes: 9765a32c ("vivid: set device_caps in video_device")

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>