mtd: maps: physmap: Invert logic on if/else branch

It is preferred to have the positive statement on an if/else. While we
are at it we replace the way we access rom_probe_types.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>