USB: serial: keyspan: handle unbound ports

Check for NULL port data in the control URB completion handlers to avoid
dereferencing a NULL pointer in the unlikely case where a port device
isn't bound to a driver (e.g. after an allocation failure on port
probe()).

Fixes: 0ca1268e ("USB Serial Keyspan: add support for USA-49WG & USA-28XG")
Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>