USB: ftdi_sio: remove obsolete check in unthrottle

No need to check ASYNCB_INITIALIZED anymore as commit
e1108a63 (usb_serial: Use the shutdown()
operation) make sure that there is no longer any call to unthrottle
after device specific close (in which the read urb is killed).

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