[media] mn88473: refactor and fix statistics

Remove DVB-T2 BER as it does not work at all and I didn't find
how to fix.

Fix DVB-T and DVB-C BER. It seems to return new some realistic
looking values.

Use (1 << 24) base for CNR calculations to keep it in line with
dvb logarithm functions.

Move all statistic logic to mn88473_read_status() function.

Use regmap_bulk_read() for reading multiple registers as a one go.

Many more and less minor changes.

Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>