mtd: ofpart: don't complain about missing 'partitions' node too loudly

The ofpart partition parser might be run on DT-enabled systems that
don't have any "ofpart" partition subnodes at all, since "ofpart" is in
the default parser list. So don't complain loudly on every boot.

Example: using m25p80.c with no intent to use ofpart:

&spi2 {
	status = "okay";

	flash@0 {
		compatible = "jedec,spi-nor";
		reg = <0>;
	};
};

I see this warning:

[    0.588471] m25p80 spi2.0: gd25q32 (4096 Kbytes)
[    0.593091] spi2.0: 'partitions' subnode not found on /spi@ff130000/flash@0. Trying to parse direct subnodes as partitions.

Cc: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>