mfd: wm831x: Fix broken wm831x_unique_id_show

wm831x_unique_id_show currently displays an interesting pattern of '0'
and '3' characters which isn't very useful (figuring out why is left
as an exercise for the reader). Presumably "buf[i]" should have been
"id[i] & 0xff".

But while there, it is much simpler to simply use %phN and do all the
formatting at once.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>