treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] 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>
diff --git a/drivers/isdn/mISDN/stack.c b/drivers/isdn/mISDN/stack.c
index 27aa329..c2f76f3 100644
--- a/drivers/isdn/mISDN/stack.c
+++ b/drivers/isdn/mISDN/stack.c
@@ -528,7 +528,7 @@ create_l2entity(struct mISDNdevice *dev, struct mISDNchannel *ch,
 		rq.protocol = ISDN_P_NT_S0;
 		if (dev->Dprotocols & (1 << ISDN_P_NT_E1))
 			rq.protocol = ISDN_P_NT_E1;
-		/* fall through */
+		fallthrough;
 	case ISDN_P_LAPD_TE:
 		ch->recv = mISDN_queue_message;
 		ch->peer = &dev->D.st->own;