tty: n_gsm: add open_error counter to gsm_mux

Extend the n_gsm link statistics by a failed link open counter in
preparation for an upcoming patch which will expose these.
This counter is increased whenever an attempt to open the control channel
failed. This is true in the following cases:
- new DLCI allocation failed
- connection request (SAMB) with invalid CR flag has been received
- connection response (UA) timed out
- parameter negotiation timed out or failed

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230817093231.2317-4-daniel.starke@siemens.com


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