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/pci/controller/pci-rcar-gen2.c

@@ -223,7 +223,7 @@ static void rcar_pci_setup(struct rcar_pci_priv *priv)
 		pr_warn("unknown window size %ld - defaulting to 256M\n",
 			window_size);
 		window_size = SZ_256M;
-		/* fall-through */
+		fallthrough;
 	case SZ_256M:
 		val |= RCAR_USBCTR_PCIAHB_WIN1_256M;
 		break;