media: i2c: adv748x: Support probing a single output

[ Upstream commit eccf442ce156ec2b4e06b1239d5fdcb0c732f63f ]

Currently the adv748x driver will fail to probe unless both of its
output endpoints (TXA and TXB) are connected.

Make the driver support probing provided that there is at least one
input, and one output connected and protect the clean-up function from
accessing un-initialized fields.

Following patches will fix other uses of un-initialized TXs in the driver,
such as power management functions.

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>