PCI: rcar-gen2: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Link: https://lore.kernel.org/r/20200722032851.GA4251@embeddedorSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>

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;