[media] media: adv7604: reduce support to first (digital) input

Using adv7611_read_cable_det() for ADV7612 means that full
support for '.max_port = ADV7604_PAD_HDMI_PORT_B,' isn't available
due to the need for multiple port reads to determine cable detection,
and an agreed mechanism for communicating the separate statuses.

This patch replaces adv7611_read_cable_det() with a functionally
identical copy, commented appropriately.

Earlier submissions [leading to commit 8331d30b] also set .cp_csc,
which is used in a cp_read() call within adv76xx_log_status().

Signed-off-by: William Towle <william.towle@codethink.co.uk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>