tty: n_gsm: name gsm tty device minors

Add a macro which defines the possible number of virtual devices for n_gsm
to improve code readability.

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220831073800.7459-2-daniel.starke@siemens.com


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