MIPS: Use fallthrough for arch/mips

Convert the various /* fallthrough */ comments to the pseudo-keyword
fallthrough;

Done via script:
https://lore.kernel.org/lkml/b56602fcf79f849e733e7b521bb0e17895d390fa.1582230379.git.joe@perches.com/

Signed-off-by: Liangliang Huang <huangll@lemote.com>
Reviewed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/alchemy/devboards/db1550.c

@@ -225,7 +225,7 @@ static void __init pb1550_nand_setup(void)
 	case 0: case 2: case 8: case 0xC: case 0xD:
 		/* x16 NAND Flash */
 		pb1550_nand_pd.devwidth = 1;
-		/* fallthrough */
+		fallthrough;
 	case 1: case 3: case 9: case 0xE: case 0xF:
 		/* x8 NAND, already set up */
 		platform_device_register(&pb1550_nand_dev);