rdtech-um: rephrase checksum verification routines

rdtech-um: rephrase checksum verification routines

Rename the unfortunately named poll_csum() callback. It neither polled
for receive data nor calculated checksums for the caller. It verifies
that a received packet's checksum is plausible, and returns a boolean.

Address coding style nits in the process. Adjust data types. Use size
types where appropriate. Don't use characters when bytes are meant. Use
booleans for the checksum test result. Shorten identifier names to trim
text line length. No need to "take the address" of a routine. Remove
dead code, model dependent checksum approaches are handled by registered
callbacks in profiles. Eliminate else after return. Prefer read_u16be()
endianess routines over RB16() preprocessor macros. Prefer lower case
hex digits. Make the code reflect that the last 16bit or 8bit entity in
the received data packet gets checked. Use "calc" and "recv" names for
awareness which variable holds which value.