media: ddbridge/mci: read and report signal strength and SNR

Implement querying signal statistics from the MCI and report this data
in read_status() as DVBv5 statistics.

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>