tty: n_gsm: add TIOCMIWAIT support

Add support for the TIOCMIWAIT ioctl on the virtual ttys. This enables the
user to wait for virtual modem signals like RING.

More work is needed to support also TIOCGICOUNT.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230206114606.2133-4-daniel.starke@siemens.com


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