tty: n_gsm: allow window size configuration

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 6 describes the error recovery mode option
which is based on I frames. The k parameter defines the maximum number of
I frames that a DLC can have unacknowledged as described in chapter 5.7.4.
The current n_gsm implementation does not support the error recovery mode
option. However, the k parameter is also part of the parameter negotiation
message as described in chapter 5.4.6.3.1. Chapter 5.7.4 also notes that
the allowed value range for k is 1-7. That means a 0 is counted as invalid
here. This means that the user needs to configure a valid value here even
if the function itself is not supported. Otherwise, parameter negotiation
may fail.

Allow setting of k via ioctl in gsm_config(). Range checks are already
included.
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230315105354.6234-2-daniel.starke@siemens.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -3276,8 +3276,8 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 	int need_close = 0;
 	int need_restart = 0;
 
-	/* Stuff we don't support yet - UI or I frame transport, windowing */
-	if ((c->adaption != 1 && c->adaption != 2) || c->k)
+	/* Stuff we don't support yet - UI or I frame transport */
+	if (c->adaption != 1 && c->adaption != 2)
 		return -EOPNOTSUPP;
 	/* Check the MRU/MTU range looks sane */
 	if (c->mru < MIN_MTU || c->mtu < MIN_MTU)