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/e2079e93f562c7f7a030eb7642017ee5eabaaa10
[2] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20201008222849.GA18634@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/musb_gadget_ep0.c

@@ -1024,7 +1024,7 @@ static int musb_g_ep0_halt(struct usb_ep *e, int value)
 	case MUSB_EP0_STAGE_ACKWAIT:	/* STALL for zero-length data */
 	case MUSB_EP0_STAGE_RX:		/* control-OUT data */
 		csr = musb_readw(regs, MUSB_CSR0);
-		/* FALLTHROUGH */
+		fallthrough;
 
 	/* It's also OK to issue stalls during callbacks when a non-empty
 	 * DATA stage buffer has been read (or even written).