media: dvb: Change mpq_streambuffer_pkt_write() to return packet index

mpq_streambuffer_pkt_write() did not return the index of the new packet
that was written. In order to get this index client would need to call
mpq_streambuffer_pkt_next() but in the mean time packet could have been
disposed and an error was returned. Changed the API so that packet
index can be retrieved without the false error.

Change-Id: I9b36343425a4e1c1a6a3b55bbfe4398e7980a1d0
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>