usb: cdc-acm: fix devices not unthrottled on open

Currently CDC-ACM devices stay throttled when their TTY is closed while
throttled, stalling further communication attempts after the next open.

Unthrottling during open/activate got lost starting with kernel
3.0.0 and this patch reintroduces it.
Signed-off-by: Otto Meta <otto.patches@sister-shadow.de>
Cc: stable <stable@vger.kernel.org>
Acked-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -567,6 +567,14 @@ static int acm_port_activate(struct tty_port *port, struct tty_struct *tty)
 
 	usb_autopm_put_interface(acm->control);
 
+	/*
+	 * Unthrottle device in case the TTY was closed while throttled.
+	 */
+	spin_lock_irq(&acm->read_lock);
+	acm->throttled = 0;
+	acm->throttle_req = 0;
+	spin_unlock_irq(&acm->read_lock);
+
 	if (acm_submit_read_urbs(acm, GFP_KERNEL))
 		goto error_submit_read_urbs;