USB: serial: ch341: handle unbound port at reset_resume

Check for NULL port data in reset_resume() to avoid dereferencing a NULL
pointer in case the port device isn't bound to a driver (e.g. after a
failed control request at port probe).

Fixes: 1ded7ea4 ("USB: ch341 serial: fix port number changed after resume")
Cc: stable <stable@vger.kernel.org>     # 2.6.30
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>