media: mb86a20s: make the bit rate estimation function more generic

While 99% of the implementation of the bitrate estimation
routine for ISDB-T is generic, the current approach mangles it
with some mb86a20s-specific thing.

Split the calculus from the specific stuff, in order to make
easier to use the same approach on other drivers requiring
a similar formula.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>