lorawan: validate requested datarates

Validate datarates requested by users. As a side effect of this change
the MAC layer is immediately aware of updated datarates. Therefore
`LoRaMacQueryTxPossible` in `lorawan_send` will be using the same
datarate to evaluate the payload length as `LoRaMacMcpsRequest` is
requesting.

Fixes #31551.

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