drivers: lora: Add support for setting sync-word and iq-inverted

Add support for setting sync-word and iq-inverted in lora_modem_config. By
being able to set the sync-word it's possible to send "raw" public-network
packets using lora_send and using iq-inverted both uplink and downlink
messages can be sent.

Signed-off-by: Tim Cooijmans <timcooijmans@gmail.com>