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>
diff --git a/drivers/ssb/driver_chipcommon.c b/drivers/ssb/driver_chipcommon.c
index 823dc99..a8d2525 100644
--- a/drivers/ssb/driver_chipcommon.c
+++ b/drivers/ssb/driver_chipcommon.c
@@ -425,7 +425,7 @@ void ssb_chipco_get_clockcontrol(struct ssb_chipcommon *cc,
 			*m = chipco_read32(cc, SSB_CHIPCO_CLOCK_M2);
 			break;
 		}
-		/* Fall through */
+		fallthrough;
 	default:
 		*m = chipco_read32(cc, SSB_CHIPCO_CLOCK_SB);
 	}