lorawan: move public network request

Move the public network request command from the end of `lorawan_start`
to the start of `lorawan_join`. This MAC command interacts with the
radio driver, bringing it out of sleep mode, and doesn't put it back.

The public network setting is only needed when joining a network, and
`lorawan_start` can be called well before `lorawan_join`. By moving the
command we save ~600uA before joining (for sx1262).

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