iio: dht11: Simplify decoding algorithm

The new algorithm uses a 'one size fits em all' threshold, which should
be easier to understand and debug. I believe there are no regressions
compared to the old adaptive threshold algorithm. I don't remember why
I chose the old algorithm when I initially wrote the driver.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>