gprs: Quectel EC21 does not understand AT+CPSB

The Quectel EC21 modem does not understand the AT+CPSB command, and we
did not find a suitable replacement in the
Quectel_EC25&EC21_AT_Commands_Manual_V1.3.pdf
AT+CPSB gives an error on this modem, so we just skip it.

drivers/atmodem/gprs.c

@@ -624,6 +624,8 @@ static void gprs_initialized(gboolean ok, GAtResult *result, gpointer user_data)
 		g_at_chat_send(gd->chat, "AT#PSNT=1", none_prefix,
 						NULL, NULL, NULL);
 		break;
+	case OFONO_VENDOR_QUECTEL_EC2X:
+		break;
 	default:
 		g_at_chat_register(gd->chat, "+CPSB:", cpsb_notify,
 						FALSE, gprs, NULL);