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/platform/olpc/olpc-xo175-ec.c b/drivers/platform/olpc/olpc-xo175-ec.c
index 5e1d14e..0d46706 100644
--- a/drivers/platform/olpc/olpc-xo175-ec.c
+++ b/drivers/platform/olpc/olpc-xo175-ec.c
@@ -431,7 +431,7 @@ static void olpc_xo175_ec_complete(void *arg)
 			input_sync(priv->pwrbtn);
 			input_report_key(priv->pwrbtn, KEY_POWER, 0);
 			input_sync(priv->pwrbtn);
-			/* fall through */
+			fallthrough;
 		case EVENT_POWER_PRESS_WAKE:
 		case EVENT_TIMED_HOST_WAKE:
 			pm_wakeup_event(priv->pwrbtn->dev.parent,