mtd: rawnand: atmel: fix OF child-node lookup

mtd: rawnand: atmel: fix OF child-node lookup

Use the new of_get_compatible_child() helper to lookup the nfc child
node instead of using of_find_compatible_node(), which searches the
entire tree from a given start node and thus can return an unrelated
(i.e. non-child) node.

This also addresses a potential use-after-free (e.g. after probe
deferral) as the tree-wide helper drops a reference to its first
argument (i.e. the node of the device being probed).

While at it, also fix a related nfc-node reference leak.

Fixes: f88fc122cc34 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Cc: stable <stable@vger.kernel.org>     # 4.11
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Josh Wu <rainyfeeling@outlook.com>
Cc: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>