media: adv748x: Store the source subdevice in TX

The power_up_tx() procedure needs to set a few registers conditionally to
the selected video source, but it currently checks for the provided tx to
be either TXA or TXB.

With the introduction of dynamic routing between HDMI and AFE entities to
TXA, checking which TX the function is operating on is not meaningful anymore.

To fix this, store the subdevice of the source providing video data to the
CSI-2 TX in the 'struct adv748x_csi2' representing the TX and check on it.

Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>