mtd: afs: break out v1 footer magic to a define

Break out the magic number to a #defined constant.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>