tty: n_gsm: mark unusable ioctl structure fields accordingly

tty: n_gsm: mark unusable ioctl structure fields accordingly

gsm_config and gsm_netconfig includes unused fields that have been included
to allow future extension without changing the structure size.
Unfortunately, no checks have been included for these field. The actual
value set by old user space code remains undefined.
This means that future extensions can not use these fields without breaking
old user space code which may set unexpected values.

Mark these fields accordingly to avoid breaking code changes.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230206114606.2133-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>