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

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: allmodconfig arm):

drivers/power/supply/ab8500_charger.c:738:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NSebastian Reichel <sebastian.reichel@collabora.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;