mtd: afs: simplify partition detection

Instead of reading out the AFS footers twice, create a separate
function to just check if there is a footer or not. Rids a few
local variables and prepare us to join the actual parser into
one function.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Richard Weinberger <richard@nod.at>