firewire: core: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/firewire/core-topology.c

@@ -58,6 +58,7 @@ static u32 *count_ports(u32 *sid, int *total_port_count, int *child_port_count)
 		case SELFID_PORT_PARENT:
 		case SELFID_PORT_NCONN:
 			(*total_port_count)++;
+			fallthrough;
 		case SELFID_PORT_NONE:
 			break;
 		}