[media] vpif_capture: get rid of some unused vars

drivers/media/platform/davinci/vpif_capture.c: In function 'vpif_channel_isr':
drivers/media/platform/davinci/vpif_capture.c:376:18: warning: variable 'field'
 set but not used [-Wunused-but-set-variable]
  enum v4l2_field field;
                  ^
drivers/media/platform/davinci/vpif_capture.c: In function 'vpif_calculate_offs
ets':
drivers/media/platform/davinci/vpif_capture.c:536:23: warning: variable 'vpitch
' set but not used [-Wunused-but-set-variable]
  unsigned int hpitch, vpitch, sizeimage;
                       ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>