media: venus: vdec: fix decoded data size

Existing code returns the max of the decoded size and buffer size.
It turns out that buffer size is always greater due to hardware
alignment requirement. As a result, payload size given to client
is incorrect. This change ensures that the bytesused is assigned
to actual payload size, when available.

Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>
Acked-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>