usb: musb: gadget: Use fallthrough pseudo-keyword

In order to enable -Wimplicit-fallthrough for Clang[1], replace the
existing /* FALLTHROUGH */ comment with the new pseudo-keyword
macro fallthrough[2].

[1] https://git.kernel.org/linus/e2079e93


[2] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20201008222849.GA18634@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>