examples/lorawan: fix semtech_loramac_send TX ret code [backport 2019.07]

[fjmolinas]

Backport of #11869

Contribution description

With #11541 (b7890b3) TX notification messages are only sent after mcps confirm event, and the code sent is SEMTECH_LORAMAC_TX_DONE instead of SEMTECH_LORAMAC_TX_OK. This PR fixes the documentation and the example accordingly.

Testing procedure

Compile and flash examples/lorawan with valid keys:

make BOARD=b-l072z-lrwan1 DEVEUI=00BD35DEE58FF9CB APPEUI=70B3D57ED000B02F APPKEY=C91870835C73F5E36EDF6E5DBD1445D0 -C examples/lorawan flash term

Without this PR TX actually succeeds but SEMTECH_LORAMAC_TX_DONE return value is perceived as error:

2019-07-19 11:54:38,531 - INFO # nding: This is Rmain(): This is RIOT! (Version: 2019.10-devel-71-ga9e72c5-pr_lorawan_retcode)
2019-07-19 11:54:38,535 - INFO # LoRaWAN Class A low-power application
2019-07-19 11:54:38,538 - INFO # =====================================
2019-07-19 11:54:38,604 - INFO # Starting join procedure
2019-07-19 11:54:43,784 - INFO # Join procedure succeeded
2019-07-19 11:54:43,786 - INFO # Sending: This is RIOT!
2019-07-19 11:54:44,902 - INFO # Cannot send message 'This is RIOT!', ret code: 6

With this PR everything is OK.

2019-07-19 11:52:59,595 - INFO # main(): This is RIOT! (Version: 2019.10-devel-71-ga9e72c5-pr_lorawan_retcode)
2019-07-19 11:52:59,599 - INFO # LoRaWAN Class A low-power application
2019-07-19 11:52:59,602 - INFO # =====================================
2019-07-19 11:52:59,668 - INFO # Starting join procedure
2019-07-19 11:53:04,849 - INFO # Join procedure succeeded
2019-07-19 11:53:04,852 - INFO # Sending: This is RIOT!
2019-07-19 11:53:25,966 - INFO # Sending: This is RIOT!

Issues/PRs references

[leandrolanzieri]

ACK.