V4L/DVB (6096): ivtv: fix V4L2_ENC_CMD_STOP_AT_GOP_END support

Support for V4L2_ENC_CMD_STOP_AT_GOP_END was broken. While the driver
correctly waited for the card to capture until the GOP was complete,
afterwards the driver buffers were just flushed instead of waiting
for the application to read all the pending data.

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