USB: Fix throttling in generic usbserial driver

The generic usbserial driver in Linux 2.6.31 halts its receiving
channel in response to throttle requests from the line discipline.
Unfortunately it drops the contents of the first URB received after
throttling takes effect. This patch corrects that problem.

Signed-off-by: Joris van Rantwijk <jorispubl@xs4all.nl>
Acked-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>