power: supply: ab8500_charger: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: allmodconfig arm):

drivers/power/supply/ab8500_charger.c: In function ‘ab8500_charger_max_usb_curr’:
drivers/power/supply/ab8500_charger.c:738:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (di->vbus_detected) {
      ^
drivers/power/supply/ab8500_charger.c:745:2: note: here
  case USB_STAT_HM_IDGND:
  ^~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

drivers/power/supply/ab8500_charger.c

@@ -742,6 +742,7 @@ static int ab8500_charger_max_usb_curr(struct ab8500_charger *di,
 						USB_CH_IP_CUR_LVL_1P5;
 			break;
 		}
+		/* Else, fall through */
 	case USB_STAT_HM_IDGND:
 		dev_err(di->dev, "USB Type - Charging not allowed\n");
 		di->max_usb_in_curr.usb_type_max = USB_CH_IP_CUR_LVL_0P05;