US compliance: oversized Echo request failure

[terrillmoore]

The RWC 5020A compliance test V1.170 section 12.2 sets the device to DR0, and sends an Echo request to port 224 with a message of 242 bytes.

The LMIC echoes the 242 byte message. At DR0 (SF10), this results in a message longer than 400 ms, which is not permitted by the FCC regulations.

The echo request should be discarded (preferably by the LMIC, as this then enforces complaint behavior on all apps using the LMIC).

This affects all regions, but only the US test exercises this.