media: i2c: adv748x: Register only enabled inputs

The adv748x assumes input endpoints are always enabled, and registers
a subdevice for each of them when the corresponding output subdevice
is registered.

Fix this by conditionally registering the input subdevice only if it is
actually described in device tree.

Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>