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

commit 6c4707f3f8c44ec18282e1c014c80e1c257042f9 upstream.

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>
Acked-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>