media: vidtv: cleanup the logic which estimates buffer size

There's no need to use u64 over there. In a matter of fact,
the div is not even needed, as it is multiplying by 1000 and
dividing by 1000.

So, simplify the logic.

While here, constrain the buffer size to a certain range
(between the current value and 10 times it)

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>