USB: airprime: unlock mutex instead of trying to lock it again

The following patch fixes a [probable] copy & paste mistake in
airprime.c. Instead of unlocking an acquired mutex, the actual
code tries to lock it again.
Signed-off-by: NLeonardo Chiquitto <lchiquitto@novell.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/airprime.c

@@ -220,7 +220,7 @@ static void airprime_close(struct usb_serial_port *port, struct file * filp)
 	mutex_lock(&port->serial->disc_mutex);
 	if (!port->serial->disconnected)
 		airprime_send_setup(port);
-	mutex_lock(&port->serial->disc_mutex);
+	mutex_unlock(&port->serial->disc_mutex);
 
 	for (i = 0; i < NUM_READ_URBS; ++i) {
 		usb_kill_urb (priv->read_urbp[i]);