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/mfd/rave-sp.c b/drivers/mfd/rave-sp.c
index abaab54..545196c 100644
--- a/drivers/mfd/rave-sp.c
+++ b/drivers/mfd/rave-sp.c
@@ -270,7 +270,7 @@ static void *stuff(unsigned char *dest, const unsigned char *src, size_t n)
 		case RAVE_SP_ETX:
 		case RAVE_SP_DLE:
 			*dest++ = RAVE_SP_DLE;
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			*dest++ = byte;
 		}
@@ -541,7 +541,7 @@ static int rave_sp_receive_buf(struct serdev_device *serdev,
 			 * deframer buffer
 			 */
 
-			/* FALLTHROUGH */
+			fallthrough;
 
 		case RAVE_SP_EXPECT_ESCAPED_DATA:
 			if (deframer->length == sizeof(deframer->data)) {