media: fsl-viu: mark static functions as such

media: fsl-viu: mark static functions as such

There are several functions that are used only inside the
driver. Stop exposing that to global symbolspace.

Get rid of the following gcc warnings:

drivers/media/platform/fsl-viu.c:240:17: warning: no previous prototype for ‘format_by_fourcc’ [-Wmissing-prototypes]
 struct viu_fmt *format_by_fourcc(int fourcc)
                 ^~~~~~~~~~~~~~~~
drivers/media/platform/fsl-viu.c:253:6: warning: no previous prototype for ‘viu_start_dma’ [-Wmissing-prototypes]
 void viu_start_dma(struct viu_dev *dev)
      ^~~~~~~~~~~~~
drivers/media/platform/fsl-viu.c:262:6: warning: no previous prototype for ‘viu_stop_dma’ [-Wmissing-prototypes]
 void viu_stop_dma(struct viu_dev *dev)
      ^~~~~~~~~~~~
drivers/media/platform/fsl-viu.c:807:5: warning: no previous prototype for ‘vidioc_g_fbuf’ [-Wmissing-prototypes]
 int vidioc_g_fbuf(struct file *file, void *priv, struct v4l2_framebuffer *arg)
     ^~~~~~~~~~~~~
drivers/media/platform/fsl-viu.c:818:5: warning: no previous prototype for ‘vidioc_s_fbuf’ [-Wmissing-prototypes]
 int vidioc_s_fbuf(struct file *file, void *priv, const struct v4l2_framebuffer *arg)
     ^~~~~~~~~~~~~
drivers/media/platform/fsl-viu.c: In function ‘viu_open’:
drivers/media/platform/fsl-viu.c:1170:6: warning: variable ‘status_cfg’ set but not used [-Wunused-but-set-variable]
  u32 status_cfg;
      ^~~~~~~~~~
drivers/media/platform/fsl-viu.c: At top level:
drivers/media/platform/fsl-viu.c:1304:6: warning: no previous prototype for ‘viu_reset’ [-Wmissing-prototypes]
 void viu_reset(struct viu_reg *reg)
      ^~~~~~~~~

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