tty: n_gsm: name the debug bits

Introduce defines to name the various debug bits used within the code to
improve readability and to make its specific use clear.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220831073800.7459-5-daniel.starke@siemens.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>