tty: n_gsm: add restart flag to DLC specific ioctl config

Currently, changing the parameters of a DLCI gives no direct control to the
user whether this should trigger a channel reset or not. The decision is
solely made by the driver based on the assumption which parameter changes
are compatible or not. Therefore, the user has no means to perform an
automatic channel reset after parameter configuration for non-conflicting
changes.

Add the parameter 'flags' to 'gsm_dlci_config' to force a channel reset
after ioctl setting regardless of whether the changes made require this or
not by setting this to 'GSM_FL_RESTART'.

Note that 'GSM_FL_RESTART' is currently the only allow flag to allow
additions here.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230817093231.2317-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>