[media] coda: simplify check in coda_buf_queue

Now that the bitstream buffer is only allocated for the BIT decoder
case, we can use bitstream.size to check for bitstream ringbuffer
operation.

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@osg.samsung.com>