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

This patch fixes the following warning:

drivers/usb/musb/musb_host.c: In function ‘musb_advance_schedule’:
drivers/usb/musb/musb_host.c:374:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (qh->mux == 1) {
       ^
drivers/usb/musb/musb_host.c:383:3: note: here
   case USB_ENDPOINT_XFER_ISOC:
   ^~~~

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

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>