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/regulator/twl6030-regulator.c b/drivers/regulator/twl6030-regulator.c
index f7db250..430265c 100644
--- a/drivers/regulator/twl6030-regulator.c
+++ b/drivers/regulator/twl6030-regulator.c
@@ -312,7 +312,7 @@ static int twl6030smps_list_voltage(struct regulator_dev *rdev, unsigned index)
 	switch (info->flags) {
 	case SMPS_OFFSET_EN:
 		voltage = 100000;
-		/* fall through */
+		fallthrough;
 	case 0:
 		switch (index) {
 		case 0: