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 <[e-mail]>
Acked-by: Charles Keepax <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/wm831x-otp.c

···
 				     struct device_attribute *attr, char *buf)
 {
 	struct wm831x *wm831x = dev_get_drvdata(dev);
-	int i, rval;
+	int rval;
 	char id[WM831X_UNIQUE_ID_LEN];
-	ssize_t ret = 0;
 
 	rval = wm831x_unique_id_read(wm831x, id);
 	if (rval < 0)
 		return 0;
 
-	for (i = 0; i < WM831X_UNIQUE_ID_LEN; i++)
-		ret += sprintf(&buf[ret], "%02x", buf[i]);
-
-	ret += sprintf(&buf[ret], "\n");
-
-	return ret;
+	return sprintf(buf, "%*phN\n", WM831X_UNIQUE_ID_LEN, id);
 }
 
 static DEVICE_ATTR(unique_id, 0444, wm831x_unique_id_show, NULL);