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/spi/spi-bcm2835aux.c

@@ -164,10 +164,10 @@ static inline void bcm2835aux_rd_fifo(struct bcm2835aux_spi *bs)
 		switch (count) {
 		case 3:
 			*bs->rx_buf++ = (data >> 16) & 0xff;
-			/* fallthrough */
+			fallthrough;
 		case 2:
 			*bs->rx_buf++ = (data >> 8) & 0xff;
-			/* fallthrough */
+			fallthrough;
 		case 1:
 			*bs->rx_buf++ = (data >> 0) & 0xff;
 			/* fallthrough - no default */