media: dvb: Use fill count for dvb_ringbuffer

For circular buffer, to distinguish between a full buffer and
an empty buffer, dvb_ringbuffer implementation reserved one
slot to always be unused (so read and write offset can
only be equal when buffer is empty).
This is not suitable for working with circular buffers
managed by hardware like TSPP2, as it causes the HW offsets
and dvb_ringbuffer offset to be out-of-sync.

Change-Id: I09749021d6d03c0f1319b358651350aa4b8dab70
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>