media: media.h: fix confusing typo in comment

Subdevs are initialized with MEDIA_ENT_F_V4L2_SUBDEV_UNKNOWN, not
MEDIA_ENT_T_V4L2_SUBDEV_UNKNOWN.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>