lorawan: make unconfirmed message type explicit

The enum did not contain an entry for unconfirmed messages. Instead, it
was only mentioned in the comment that 0 is the default for
unconfirmed messages.

This commit adds LORAWAN_MSG_UNCONFIRMED to the enum and changes the
parameter in the lorawan_send function to enum lorawan_message_type.

Signed-off-by: Martin Jäger <martin@libre.solar>