media: bttv: radio use v4l2_fh instead of bttv_fh

Use a v4l2_fh when opening a radio device instead of a bttv_fh and manage
it with v4l2_fh_open() and v4l2_fh_release() and v4l2_ctrl_poll(). This
eliminates bttv_fh from the radio in preparation for vb2 conversion which
stops using separate bttv file handles altogether.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>