[nrf fromlist] drivers: modem_cellular: fix handling of +C*REG answers

Answers to the `AT+CEREG` read command may contain many parameters
depending on how it is configured (with the write command).
When it has more than one parameter, the `<stat>` one (which we are
interested in) is at the second position.
This is the syntax defined by the 3GPP spec for all `+C*REG` commands.
Handle all parameter counts properly.

Signed-off-by: Tomi Fontanilles <[email protected]>
(cherry picked from commit 5876b371032a16af2f12469c7f23fbbb5594d333)

Upstream PR #: 80512

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

Author: tomi-font

drivers/modem/modem_cellular.c

@@ -456,7 +456,7 @@ static void modem_cellular_chat_on_cxreg(struct modem_chat *chat, char **argv, u
 
 	if (argc == 2) {
 		registration_status = atoi(argv[1]);
-	} else if (argc == 3 || argc == 6) {
+	} else if (argc >= 3) {
 		registration_status = atoi(argv[2]);
 	} else {
 		return;