The prefix suggests the number should be printed in hex, so use
the %x specifier to do that.

Found by using regex suggested by Joe Perches.
Signed-off-by: Hans Wennborg <hans@hanshq.net>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>