mux: core: fix double get_device()

class_find_device already does a get_device on the returned device.
So the device returned by of_find_mux_chip_by_node is already referenced
and we should not reference it again (and unref it on error).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>