b43legacy: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821064412.GA20612@embeddedor

drivers/net/wireless/broadcom/b43legacy/dma.c

@@ -189,7 +189,7 @@ return dev->dma.tx_ring1;
 	switch (queue_priority) {
 	default:
 		B43legacy_WARN_ON(1);
-		/* fallthrough */
+		fallthrough;
 	case 0:
 		ring = dev->dma.tx_ring3;
 		break;

drivers/net/wireless/broadcom/b43legacy/main.c

@@ -1538,7 +1538,7 @@ static int do_request_fw(struct b43legacy_wldev *dev,
 		size = be32_to_cpu(hdr->size);
 		if (size != (*fw)->size - sizeof(struct b43legacy_fw_header))
 			goto err_format;
-		/* fallthrough */
+		fallthrough;
 	case B43legacy_FW_TYPE_IV:
 		if (hdr->ver != 1)
 			goto err_format;
@@ -2077,7 +2077,7 @@ static void b43legacy_rate_memory_init(struct b43legacy_wldev *dev)
 		b43legacy_rate_memory_write(dev, B43legacy_OFDM_RATE_36MB, 1);
 		b43legacy_rate_memory_write(dev, B43legacy_OFDM_RATE_48MB, 1);
 		b43legacy_rate_memory_write(dev, B43legacy_OFDM_RATE_54MB, 1);
-		/* fallthrough */
+		fallthrough;
 	case B43legacy_PHYTYPE_B:
 		b43legacy_rate_memory_write(dev, B43legacy_CCK_RATE_1MB, 0);
 		b43legacy_rate_memory_write(dev, B43legacy_CCK_RATE_2MB, 0);