software node: Allow node addition to already existing device

If the node is added to an already exiting device, the node
needs to be also linked to the device separately.

This will make sure the reference count is kept in balance
also when the node is injected to a device afterwards.

Fixes: e68d0119

 ("software node: Introduce device_add_software_node()")
Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210414075438.64547-1-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>