rdtech-tc: grab receive data in larger chunks

rdtech-tc: grab receive data in larger chunks

The previous implementation used to read each of the 192 bytes in
individual calls into the serial layer. This implementation grabs
receive data in larger chunks, and accepts data to be in flight
between main loop callbacks.

USB CDC appears to reliably deliver 64 byte chunks (unsurprisingly).
BLE communication is yet to get tested, isn't operational with current
mainline BT support because MTU exchange is not implemented.

(Re-)synchronization to the stream of packets after hiccups remains
an option which this implementation does not provide, neither did
the previous implementation.