Revert "media: dvb: Use fill count for dvb_ringbuffer"

This reverts commit f3ba3f6a.

In order to sync on buffer state with SDMX, demux manipulates the
read/write pointers of dvb_ringbuffer directly to account for new
data from SDMX. When dvb_ringbuffer is using fill count internally,
updating the read/write pointers does not affect the fill count
and so buffer is out of sync with SDMX.

Change-Id: I70828ba3337e49c53da67862b20d5f2c119f4271
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>