When sending a command to the CC13xx/CC25xx RF core, we wait for command completion by checking the LSB of CMDSTA (correctly so). However, in doing so we also zero out the 3 CMDSTA return bytes. For some commands, those bytes contain useful information (e.g. an RSSI value) and are required by the caller.
This problem manifests itself e.g. in PROP mode `channel_clear()`, whereby the caller will always see an RSSI value of 0.
This pull therefore fixes the logic in `rf_core_send_cmd()` to check for command completion by blocking on the CMDSTA result byte without zeroing out the 3 return bytes.
Fixes#1465
d42b1b50e5