media: ddbridge/mci: add SX8 I/Q mode remark and remove DIAG CMD defines

Take note that the SX8 IQ mode is only available on a single tuner, and
remove the MCI/SX8 DIAG CMD defines.

Picked up from the upstream dddvb GIT.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>