tty: n_gsm: fix wrong T1 retry count handling

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 5.7.3 states that the valid range for the
maximum number of retransmissions (N2) is from 0 to 255 (both including).
gsm_dlci_t1() handles this number incorrectly by performing N2 - 1
retransmission attempts. Setting N2 to zero results in more than 255
retransmission attempts.
Fix gsm_dlci_t1() to comply with 3GPP 27.010.

Fixes: e1eaea46 ("tty: n_gsm line discipline")
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220707113223.3685-1-daniel.starke@siemens.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -1805,8 +1805,8 @@ static void gsm_dlci_t1(struct timer_list *t)
 
 	switch (dlci->state) {
 	case DLCI_OPENING:
-		dlci->retries--;
 		if (dlci->retries) {
+			dlci->retries--;
 			gsm_command(dlci->gsm, dlci->addr, SABM|PF);
 			mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
 		} else if (!dlci->addr && gsm->control == (DM | PF)) {
@@ -1821,8 +1821,8 @@ static void gsm_dlci_t1(struct timer_list *t)
 
 		break;
 	case DLCI_CLOSING:
-		dlci->retries--;
 		if (dlci->retries) {
+			dlci->retries--;
 			gsm_command(dlci->gsm, dlci->addr, DISC|PF);
 			mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
 		} else