V4L/DVB (3484): Make dvb_ringbuffer compatible to dmxdev_buffer

Added variable 'error' to struct dvb_ringbuffer, which is set to zero on
init() and flush(). Also reset read an write pointers to zero on flush()
to get less fragmented data.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>