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/include/asm/octeon/cvmx-sli-defs.h

@@ -46,7 +46,7 @@ static inline uint64_t CVMX_SLI_PCIE_MSI_RCV_FUNC(void)
 	case OCTEON_CN78XX & OCTEON_FAMILY_MASK:
 		if (OCTEON_IS_MODEL(OCTEON_CN78XX_PASS1_X))
 			return 0x0000000000003CB0ull;
-		/* Else, fall through */
+		fallthrough;
 	default:
 		return 0x0000000000023CB0ull;
 	}