mfd: core: Preserve OF node when ACPI handle is present

Switch device_set_node to set_primary_fwnode, so that the ACPI fwnode
does not overwrite the of_node with NULL.

This allows MFD children with both OF nodes and ACPI handles to have OF
nodes again.

Cc: stable@vger.kernel.org
Fixes: 51e3b257099d ("mfd: core: Make use of device_set_node()")
Signed-off-by: Brian Mak <makb@juniper.net>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260325223024.35992-1-makb@juniper.net
Signed-off-by: Lee Jones <lee@kernel.org>