drivers: lora: lbm: Defer radio initialization until first config

Split device initialization into two phases across all LBM drivers:

1. Boot-time init: Minimal device initialization
2. First config: Radio hardware initialization

This deferred initialization approach provides full control over the
radio hardware, allowing applications to perform any necessary setup
before the radio is initialized. The radio init is automatically
triggered on the first call to lora_config().

Signed-off-by: Carlo Caione <ccaione@baylibre.com>