V4L/DVB (3500): Ringbuffer: don't reset pointers to zero

Oliver Endriss spotted, that resetting read and write pointers on
flush() requires additional locking and breaks the av7110 driver.
Therefore this patch partially reverts the previous patch titled "make
dvb_ringbuffer compatible to dmxdev_buffer".

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