media: v4l2-fwnode: make v4l2_fwnode_endpoint_parse_csi1_bus static

This function is used only internally. So, make it static.
Shuts up this warning:

drivers/media/v4l2-core/v4l2-fwnode.c:153:6: warning: no previous prototype for 'v4l2_fwnode_endpoint_parse_csi1_bus' [-Wmissing-prototypes]
 void v4l2_fwnode_endpoint_parse_csi1_bus(struct fwnode_handle *fwnode,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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