Set RX1 frequency in config for logging

[johanstokking]

Description

The logged frequency of the open RX1 slot is wrong at this line; https://github.com/ARMmbed/mbed-os/blob/master/features/lorawan/lorastack/mac/LoRaMac.cpp#L910

This is because the channel is filled out, but not the frequency. Only rx_config() should know the frequency (as it knows the frequency plan and RX1 DR offsets), so this change sets the frequency in there, so it gets logged correctly.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@hasnainvirk @janjongboom

Release Notes

[ciarmcom]

@johanstokking, thank you for your changes.
@janjongboom @hasnainvirk @ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[hasnainvirk]

@johanstokking I like this. I had already spotted that while doing a related task. #10214 This PR is taking care of what you need. Would that suit your needs ?

[johanstokking]

Thanks @hasnainvirk, indeed #10214 already covers this.