usb: use usb_serial_put in usb_serial_probe errors

The use of kfree(serial) in error cases of usb_serial_probe
was invalid - usb_serial structure allocated in create_serial()
gets reference of usb_device that needs to be put, so we need
to use usb_serial_put() instead of simple kfree().

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