mfd: core: Set fwnode for created devices

The logic for setting the of_node on devices created by mfd did not set
the fwnode pointer to match, which caused fwnode-based APIs to
malfunction on these devices since the fwnode pointer was null. Fix
this.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: Lee Jones <lee.jones@linaro.org>