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/net/ethernet/microchip/lan743x_ethtool.c b/drivers/net/ethernet/microchip/lan743x_ethtool.c
index c533d06..dcde496 100644
--- a/drivers/net/ethernet/microchip/lan743x_ethtool.c
+++ b/drivers/net/ethernet/microchip/lan743x_ethtool.c
@@ -548,7 +548,7 @@ static int lan743x_ethtool_get_rxnfc(struct net_device *netdev,
 		case TCP_V4_FLOW:case UDP_V4_FLOW:
 		case TCP_V6_FLOW:case UDP_V6_FLOW:
 			rxnfc->data |= RXH_L4_B_0_1 | RXH_L4_B_2_3;
-			/* fall through */
+			fallthrough;
 		case IPV4_FLOW: case IPV6_FLOW:
 			rxnfc->data |= RXH_IP_SRC | RXH_IP_DST;
 			return 0;