mtd: afs: refactor v1 partition parsing

Return immediately if we are not finding a valid v1 partition
in afs_read_footer_v1(), invert scanning logic so we continue
to read image information on v1 if we found a footer. This is
needed for the logic we introduce to parse v2 footers.

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>