software node: fix wrong node passed to find nargs_prop

nargs_prop refers to a property located in the reference that is found
within the nargs property. Use the correct reference node in call to
property_entry_read_int_array() to retrieve the correct nargs value.

Fixes: b06184ac ("software node: Add software_node_get_reference_args()")
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>