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>