[media] pxa_camera: remove an unused structure pointer

As reported by smatch:

drivers/media/platform/pxa_camera.c: In function 'pxa_dma_start_channels':
drivers/media/platform/pxa_camera.c:457:21: warning: variable 'active' set but not used [-Wunused-but-set-variable]
  struct pxa_buffer *active;
                     ^~~~~~

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