[media] coda: get rid of unused var

Some vars are not used, as warned by gcc:

drivers/media/platform/coda/coda-common.c: In function 'coda_buf_is_end_of_stream':
drivers/media/platform/coda/coda-common.c:816:20: warning: variable 'src_vq' set but not used [-Wunused-but-set-variable]
  struct vb2_queue *src_vq;
                    ^~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>