A 188 byte gap has to be left between the writer and the consumer. This
requires 2*188 bytes available to be able to write to the output buffers.
So, change ddb_output_free() to report free bytes according to this rule.

Picked up from the upstream dddvb-0.9.33 release.
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>