media: tegra-video: Fix compilation warning of unused variable

vi_pattern_strings is used only when CONFIG_VIDEO_TEGRA_TPG is
enabled and V4L2 control operations currently are used only in
TPG mode.

So when tegra-video is build for non TPG, warnings of unused
variable is reported for v4l2 control operation variable.

This patch fixes it.

Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Sowjanya Komatineni <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/staging/media/tegra-video/vi.c

···
 /*
  * V4L2 control operations
  */
+#if IS_ENABLED(CONFIG_VIDEO_TEGRA_TPG)
 static int vi_s_ctrl(struct v4l2_ctrl *ctrl)
 {
 	struct tegra_vi_channel *chan = container_of(ctrl->handler,
···
 	"Black/White Direct Mode",
 	"Color Patch Mode",
 };
+#endif
 
 static int tegra_channel_setup_ctrl_handler(struct tegra_vi_channel *chan)
 {