sx127x: 2 fixes and 1 enhancement (replaces #10917)

[korotkoves]

Contribution description

1 bugfix, 1 fix and 1 enhancement for sx127x driver (sx1272/sx1276 Lora chipset)

Bugfix for sending in implicit headers mode
Fix for netdev send call (remove duplicate code + bugfix for sx127x_set_tx())
Enhancement: set chipset's ValidHeader interrupt to NETDEV_EVENT_RX_STARTED netdev event

Testing procedure

To test enhancement, add NETDEV_EVENT_RX_STARTED catching code in event callback function

Issues/PRs references

Replaces PR #10917

[aabadie]

I just tried this PR and I can confirm that RX started is correctly triggered, pkg_semtech-loramac also still works.
ValidHeader detection irq is not enabled in the original Semtech driver (see sx1272 and sx1276) but it's just that it doesn't care about RX started events.
I just amended a few minor style issues and pushed 36affe1 to avoid having the message "Unexpected netdev event received" in tests/driver_sx127x application each time a message is received.

For me, this PR is good to go but maybe @dylad, @jia200x or @leandrolanzieri are also interested in having a look ?

[jia200x]

the changes make sense to me. I will test it with #11022

[jia200x]

the changes make sense to me. I will test it with #11022

It works fine with #11022.

[aabadie]

Thanks for testing @jia200x, let's merge this one.

ACK and go