USB: serial: mct_u232: fix modem-status error handling

commit 36356a66 upstream.

Make sure to detect short control-message transfers so that errors are
logged when reading the modem status at open.

Note that while this also avoids initialising the modem status using
uninitialised heap data, these bits could not leak to user space as they
are currently not used.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/mct_u232.c

@@ -385,9 +385,13 @@ static int mct_u232_get_modem_stat(struct usb_serial *serial,
 			MCT_U232_GET_REQUEST_TYPE,
 			0, 0, buf, MCT_U232_GET_MODEM_STAT_SIZE,
 			WDR_TIMEOUT);
-	if (rc < 0) {
+	if (rc < MCT_U232_GET_MODEM_STAT_SIZE) {
 		dev_err(&serial->dev->dev,
 			"Get MODEM STATus failed (error = %d)\n", rc);
+
+		if (rc >= 0)
+			rc = -EIO;
+
 		*msr = 0;
 	} else {
 		*msr = buf[0];