usb: musb: add code comment for clarification

Add code comment to make it clear that the fall-through is intentional.
Read the link for more details: https://lkml.org/lkml/2017/2/9/292



Addresses-Coverity-ID: 1397608
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>