[media] tw5864: remove two unused vars

[media] tw5864: remove two unused vars

Remove those two vars that aren't used, as reported by smatch:

drivers/media/pci/tw5864/tw5864-video.c: In function 'tw5864_prepare_frame_headers':
drivers/media/pci/tw5864/tw5864-video.c:1219:16: warning: variable 'space_before_sl_hdr' set but not used [-Wunused-but-set-variable]
  unsigned long space_before_sl_hdr;
                ^~~~~~~~~~~~~~~~~~~
drivers/media/pci/tw5864/tw5864-video.c:1218:6: warning: variable 'sl_hdr' set but not used [-Wunused-but-set-variable]
  u8 *sl_hdr;
      ^~~~~~

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