mtd: use dev_of_node helper in mtd_get_of_node

This allows better compile-time optimizations with CONFIG_OF disabled.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>