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/cpufreq/speedstep-lib.c

@@ -366,7 +366,7 @@ enum speedstep_processor speedstep_detect_processor(void)
 			} else
 				return SPEEDSTEP_CPU_PIII_C;
 		}
-		/* fall through */
+		fallthrough;
 	default:
 		return 0;
 	}