[media] mb86a20s: add BER measurement

Add the methods to read bit error/bit count measurements from
mb86a20s. On ISDB-T devices, those reads are done per layer.
However, as userspace applications may not be aware of that,
add a global measure that will sum the bit errors and bit
counts for each layer, storing them into a global value.

Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>