sensor: dht: Fix multi-instance type handling

The DHT driver incorrectly assumes all sensor instances are the same
type as the first instance (dht@0). The data parsing logic uses a
hardcoded check of the devicetree property for instance 0, which
causes incorrect readings when different sensor models (e.g., DHT11
and DHT22) are used together.

This change stores the model type in each per-instance config struct
during initialization. The data parsing logic is updated to use this
per-instance flag, ensuring each sensor is handled correctly
according to its specific model.

Signed-off-by: John Shelton <moosery@gmail.com>