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>

drivers/net/ethernet/smsc/smc911x.c

@@ -712,7 +712,7 @@ static void smc911x_phy_detect(struct net_device *dev)
 					/* Found an external PHY */
 					break;
 			}
-			/* Else, fall through */
+			fallthrough;
 		default:
 			/* Internal media only */
 			SMC_GET_PHY_ID1(lp, 1, id1);