Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/usb/phy/phy-ab8500-usb.c: In function 'ab8500_usb_link_status_update':
drivers/usb/phy/phy-ab8500-usb.c:424:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   event = UX500_MUSB_RIDB;
   ~~~~~~^~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-ab8500-usb.c:425:2: note: here
  case USB_LINK_NOT_CONFIGURED_8500:
  ^~~~
drivers/usb/phy/phy-ab8500-usb.c:440:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   event = UX500_MUSB_RIDC;
   ~~~~~~^~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-ab8500-usb.c:441:2: note: here
  case USB_LINK_STD_HOST_NC_8500:
  ^~~~
drivers/usb/phy/phy-ab8500-usb.c:459:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   event = UX500_MUSB_RIDA;
   ~~~~~~^~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-ab8500-usb.c:460:2: note: here
  case USB_LINK_HM_IDGND_8500:
  ^~~~
drivers/usb/phy/phy-ab8500-usb.c: In function 'ab8505_usb_link_status_update':
drivers/usb/phy/phy-ab8500-usb.c:332:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   event = UX500_MUSB_RIDB;
   ~~~~~~^~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-ab8500-usb.c:333:2: note: here
  case USB_LINK_NOT_CONFIGURED_8505:
  ^~~~
drivers/usb/phy/phy-ab8500-usb.c:352:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   event = UX500_MUSB_RIDC;
   ~~~~~~^~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-ab8500-usb.c:353:2: note: here
  case USB_LINK_STD_HOST_NC_8505:
  ^~~~
drivers/usb/phy/phy-ab8500-usb.c:370:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   event = UX500_MUSB_RIDA;
   ~~~~~~^~~~~~~~~~~~~~~~~
drivers/usb/phy/phy-ab8500-usb.c:371:2: note: here
  case USB_LINK_HM_IDGND_8505:
  ^~~~
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20190729000631.GA24165@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>