In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

In file included from drivers/usb/usbip/vhci_hcd.c:15:
drivers/usb/usbip/vhci_hcd.c: In function ‘vhci_hub_control’:
drivers/usb/usbip/usbip_common.h:63:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (flag & usbip_debug_flag)  \
      ^
drivers/usb/usbip/usbip_common.h:77:2: note: in expansion of macro ‘usbip_dbg_with_flag’
  usbip_dbg_with_flag(usbip_debug_vhci_rh, fmt , ##args)
  ^~~~~~~~~~~~~~~~~~~
drivers/usb/usbip/vhci_hcd.c:509:4: note: in expansion of macro ‘usbip_dbg_vhci_rh’
    usbip_dbg_vhci_rh(
    ^~~~~~~~~~~~~~~~~
drivers/usb/usbip/vhci_hcd.c:511:3: note: here
   case USB_PORT_FEAT_U2_TIMEOUT:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>