rdtech-tc: request keyword differs between CDC and BLE transports

rdtech-tc: request keyword differs between CDC and BLE transports

The TC66C firmware implements different keywords and different request
terminators across the USB CDC and the Bluetooth LE transports. Move the
string literal to the probe routine, construct the request text at the
time when the device is probed, then re-use it for the remaining session.

Check the receive buffer size and maximum reponse length for consistency
at compile time. Declarations are spread across source files.