drivers: lora: fix busy after sx12xx_lora_test_cw

The LoRa modem has been staying in STATE_BUSY
after sx12xx_lora_test_cw() call
because Radio.SetTxContinuousWave() doesn't configure the PHY
to raise 'TX done' event.

It runs TxTimeoutTimer instead,
so we have to handle 'TX timeout' event to correctly release the modem.

This fix has been tested on custom board with STM32L4 and SX1276.

Signed-off-by: Petr Sharshavin <sharshavin@mail.ru>