mtd: physmap_of: Fix ROM support via OF

The "ROM" and unknown probe types within the obsolete "direct-mapped"
probe function used the nonexistent "mtd_rom" probe instead of the
intended "map_rom".

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>