USB: mos7840: fix race in led handling

USB: mos7840: fix race in led handling

commit 05cf0dec5ccc696a7636c84b265b477173498156 upstream.

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.

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