lorawan: restore datarate after join

Restore the user requested datarate upon a successful join. Several
regions overwrite the configured datarate through `RegionAlternateDr`,
which means that for these regions, `LoRaMacQueryTxPossible` will not
be evaluating the requested datarate for the first transmission after
joining.

Fixes #31551.

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