atmodem: Deactivate AT+CPSB for quectel serial modems

There are at the moment two quectel modems in ofono vendored as
OFONO_VENDOR_QUECTEL_SERIAL: The M95 and the MC60.
Both modems are GSM only modems, and their official documentation does
not mention the AT+CPSB command.
I have a M95 here that gives an error on issuing the AT+CPSB=1 command.
So skip this command for these two modems.

drivers/atmodem/gprs.c

@@ -625,6 +625,7 @@ static void gprs_initialized(gboolean ok, GAtResult *result, gpointer user_data)
 						NULL, NULL, NULL);
 		break;
 	case OFONO_VENDOR_QUECTEL_EC2X:
+	case OFONO_VENDOR_QUECTEL_SERIAL:
 		break;
 	default:
 		g_at_chat_register(gd->chat, "+CPSB:", cpsb_notify,