media: dvb: Fix zero write to input pipe

Processing of the input buffer is done with rounding down of data
length to complete TS packet size. Zero data size might be requested
which will not be performed by BAM and so no completion callback will
occur which will cause processing of input buffer to stop.

Change-Id: I1fda140159eb1eb8bb0f9bce770fbc87af72fadd
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>