USB: ftdi_sio: fix initialisation of latency timeout

Latency timeout was read but never stored on port probe. When
ASYNC_LOW_LATENCY was cleared the device timeout would get set to 0
rather than the default 16ms.

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