drivers: modem_cellular: fix handling of +C*REG answers

`+C*REG:` may be received as AT read command answer
or unsolicited notification.
Their syntax differs, and even the overall parameter count varies
depending on what `<n>` is used in the `AT+CEREG=` write command.

To handle all cases properly, check the parameter count and
the presence of the `<tac>` parameter (which is a string and
thus begins with `"`) to figure out what is the position of `<stat>`.

Signed-off-by: Tomi Fontanilles <[email protected]>

Author: tomi-font

drivers/modem/modem_cellular.c

@@ -460,10 +460,15 @@ static void modem_cellular_chat_on_cxreg(struct modem_chat *chat, char **argv, u
 	struct modem_cellular_data *data = (struct modem_cellular_data *)user_data;
 	enum cellular_registration_status registration_status = 0;
 
-	if (argc == 2) {
-		registration_status = atoi(argv[1]);
-	} else if (argc == 3 || argc == 6) {
+	/* This receives both +C*REG? read command answers and unsolicited notifications.
+	 * Their syntax differs in that the former has one more parameter, <n>, which is first.
+	 */
+	if (argc >= 3 && argv[2][0] != '"') {
+		/* +CEREG: <n>,<stat>[,<tac>[...]] */
 		registration_status = atoi(argv[2]);
+	} else if (argc >= 2) {
+		/* +CEREG: <stat>[,<tac>[...]] */
+		registration_status = atoi(argv[1]);
 	} else {
 		return;
 	}
@@ -472,7 +477,7 @@ static void modem_cellular_chat_on_cxreg(struct modem_chat *chat, char **argv, u
 		data->registration_status_gsm = registration_status;
 	} else if (strcmp(argv[0], "+CGREG: ") == 0) {
 		data->registration_status_gprs = registration_status;
-	} else {
+	} else { /* CEREG */
 		data->registration_status_lte = registration_status;
 	}