V4L/DVB (6087): ivtv: prevent changing VBI format while capture is in progress

Changing the VBI format requires a CX2341X_ENC_INITIALIZE_INPUT firmware
call. This can only be done if no capture is in progress. So return
-EBUSY if the encoder is busy.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>