FSK downlink needs fdev set

[tftelkamp]

Please add fdev to the FSK txpk, with a value of 25000.

ipol you can remove (it only applies to LORA).

JSON down: {"txpk":{"imme":false,"tmst":1082005352,"freq":868.8,"rfch":0,"powe":14,"modu":"FSK","datr":50000,"ipol":false,"size":12,"data":"YFR4RQIgAACSQU77"}}
WARNING: [down] no mandatory "txpk.fdev" object in JSON, TX aborted

[tftelkamp]

Addendum.

For a (G)FSK modulation index of 1 (as specific in AN1200.24), the frequency deviation (fdev) is half of the bitrate. Hence the 25000.

However, LoRaWAN only specifies an FSK bitrate of 50kbps. So maybe it's better regardless of the uplink datarate to always reply with datarate 50000 and fdev 25000.

[htdvisser]

See also https://github.com/TheThingsNetwork/lora-gateway-bridge/commit/78d163b68ef818357b837f7f6a15f4d6dd60d624#diff-c1f2b900561bef280eae8703e2a1329cR459