lorawan: store current datarate for ADR

Keep track of the current network datarate when ADR is enabled.

The datarate can change in two circumstances. Firstly, the datarate may
change as a result of a command from the LoRaWAN server as the link
budget varies. Secondly, the datarate may be changed due to not
receiving an expected ADRACKReq response. This necessitates querying the
datarate on both packet reception and transmission to provide timely
notifications to the application.

Due to querying the default region datarate at startup and validating
manual datarate parameters, when ADR is not enabled the datarate will
never be different from the value provided to `lorawan_set_datarate`.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>