power: supply: twl4030_charger: fix charging current out-of-bounds

the charging current uses unsigned int variables, if we step back
if the current is still low, we would run into negative which
means setting the target to a huge value.
Better add checks here.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/supply/twl4030_charger.c

@@ -420,7 +420,8 @@ static void twl4030_current_worker(struct work_struct *data)
 
 	if (v < USB_MIN_VOLT) {
 		/* Back up and stop adjusting. */
-		bci->usb_cur -= USB_CUR_STEP;
+		if (bci->usb_cur >= USB_CUR_STEP)
+			bci->usb_cur -= USB_CUR_STEP;
 		bci->usb_cur_target = bci->usb_cur;
 	} else if (bci->usb_cur >= bci->usb_cur_target ||
 		   bci->usb_cur + USB_CUR_STEP > USB_MAX_CURRENT) {