usb: fix issue when USB host ask for string descriptor and USB device answer with exact 128 bytes.

BugLink: http://bugs.launchpad.net/bugs/611634

In this case USB host don.t know if transaction is finished

Signed-off-by: Eric Miao <eric.miao@canonical.com>
Signed-off-by: Brad Figg <brad.figg@canonical.com>