pinctrl: Avoid coverity warning when checking width

The width is set up in single_of_to_plat() and can only have three values,
all of which result in a non-zero divisor. Add a comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 331154)
diff --git a/drivers/pinctrl/pinctrl-single.c b/drivers/pinctrl/pinctrl-single.c
index 7af6c5f..cf9ad36 100644
--- a/drivers/pinctrl/pinctrl-single.c
+++ b/drivers/pinctrl/pinctrl-single.c
@@ -471,6 +471,7 @@
 		return -ENOMEM;
 	#endif
 
+	/* looks like a possible divide by 0, but data->width avoids this */
 	priv->npins = size / (pdata->width / BITS_PER_BYTE);
 	if (pdata->bits_per_mux) {
 		if (!pdata->mask) {