USB: mos7840: fix race in led handling

Fix race in LED handling introduced by commit 0eafe4de ("USB: serial:
mos7840: add support for MCS7810 devices") which reused the port control
urb for manipulating the LED without making sure that the urb is not
already in use. This could lead to the control urb being manipulated
while in flight.

Fix by adding a dedicated LED urb and ctrlrequest along with a LED-busy
flag to handle concurrency.

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