device property: Return -ENXIO if there is no suitable FW interface

Return -ENXIO if device property array access functions don't find
a suitable firmware interface.

This lets drivers decide if they should use available platform data
instead.

Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>