v4l2object: Use non-deprecated V4L2 type for RGB15

Support for the updated V4L2_PIX_FMT_XRGB555 was added in commit
2538fee2fd however, when setting the format
for use in v4l2 ioctls, the old deprecated format is still used. Convert
this to the new accepted format type, as the preferred format.

https://bugzilla.gnome.org/show_bug.cgi?id=767300

sys/v4l2/gstv4l2object.c

@@ -1661,6 +1661,7 @@ gst_v4l2_object_get_caps_info (GstV4l2Object * v4l2object, GstCaps * caps,
         break;
       case GST_VIDEO_FORMAT_RGB15:
         fourcc = V4L2_PIX_FMT_RGB555;
+        fourcc_nc = V4L2_PIX_FMT_XRGB555;
         break;
       case GST_VIDEO_FORMAT_RGB16:
         fourcc = V4L2_PIX_FMT_RGB565;