tty: n_gsm: add ioctl for DLC specific parameter configuration

tty: n_gsm: add ioctl for DLC specific parameter configuration

Parameter negotiation has been introduced with
commit 92f1f0c3290d ("tty: n_gsm: add parameter negotiation support")

However, means to set individual parameters per DLCI are not yet
implemented. Furthermore, it is currently not possible to keep a DLCI half
open until the user application sets the right parameters for it. This is
required to allow a user application to set its specific parameters before
the underlying link is established. Otherwise, the link is opened and
re-established right afterwards if the user application sets incompatible
parameters. This may be an unexpected behavior for the peer.

Add parameter 'wait_config' to 'gsm_config' to support setups where the
DLCI specific user application sets its specific parameters after open()
and before the link gets fully established. Setting this to zero disables
the user application specific DLCI configuration option.

Add the ioctls 'GSMIOC_GETCONF_DLCI' and 'GSMIOC_SETCONF_DLCI' for the
ldisc and virtual ttys. This gets/sets the DLCI specific parameters and may
trigger a reconnect of the DLCI if incompatible values have been set. Only
the parameters for the DLCI associated with the virtual tty can be set or
retrieved if called on these.

Add remark within the documentation to introduce the new ioctls.

Link: https://lore.kernel.org/oe-kbuild-all/202302281856.S9Lz4gHB-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230315105354.6234-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>