media: atomisp: Properly initialize function field of media-entity links

Don't use MEDIA_ENT_F_V4L2_SUBDEV_UNKNOWN to initialize the function field
of various media-entity links.

This fixes the following warnings showing up in dmesg:

 atomisp-isp2 0000:00:03.0: Entity type for entity ATOM ISP CSI2-port0 was not initialized!
 atomisp-isp2 0000:00:03.0: Entity type for entity ATOM ISP CSI2-port1 was not initialized!
 atomisp-isp2 0000:00:03.0: Entity type for entity ATOM ISP CSI2-port2 was not initialized!
 atomisp-isp2 0000:00:03.0: Entity type for entity tpg_subdev was not initialized!
 atomisp-isp2 0000:00:03.0: Entity type for entity ATOMISP_SUBDEV_0 was not initialized!
 atomisp-isp2 0000:00:03.0: Entity type for entity ATOMISP_SUBDEV_1 was not initialized!

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>