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: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>