iio: dht11: Improve detection of sensor type

The old code was based on a DHT11 datasheet which specifies a measurement
range of 20%-90% RH. Turns out the sensor actually reports values outside
this range, so we should support it as far as possible.

Reported-by: Edward Attfield <edward@attfield.ca>
Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>