[media] coda: stop setting bytesused in buf_prepare

The application must have filled the bytesused field,
don't overwrite it here.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>