mtd: maps: physmap: Fix coding style issues reported by checkpatch

Fix the following coding style issues:
- != NULL and == NULL test replaced by ! (or nothing)
- split over 80 chars lines
- add missing braces in multi-line if() {} else {} statements

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