rtl8xxxu: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix
multiple warnings by replacing /* fall through */ comments with
the new pseudo-keyword macro fallthrough; instead of letting the
code fall through to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210305094850.GA141221@embeddedor

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -1145,7 +1145,7 @@ void rtl8xxxu_gen1_config_channel(struct ieee80211_hw *hw)
 	switch (hw->conf.chandef.width) {
 	case NL80211_CHAN_WIDTH_20_NOHT:
 		ht = false;
-		/* fall through */
+		fallthrough;
 	case NL80211_CHAN_WIDTH_20:
 		opmode |= BW_OPMODE_20MHZ;
 		rtl8xxxu_write8(priv, REG_BW_OPMODE, opmode);
@@ -1272,7 +1272,7 @@ void rtl8xxxu_gen2_config_channel(struct ieee80211_hw *hw)
 	switch (hw->conf.chandef.width) {
 	case NL80211_CHAN_WIDTH_20_NOHT:
 		ht = false;
-		/* fall through */
+		fallthrough;
 	case NL80211_CHAN_WIDTH_20:
 		rf_mode_bw |= WMAC_TRXPTCL_CTL_BW_20;
 		subchannel = 0;
@@ -1741,11 +1741,11 @@ static int rtl8xxxu_identify_chip(struct rtl8xxxu_priv *priv)
 		case 3:
 			priv->ep_tx_low_queue = 1;
 			priv->ep_tx_count++;
-			/* fall through */
+			fallthrough;
 		case 2:
 			priv->ep_tx_normal_queue = 1;
 			priv->ep_tx_count++;
-			/* fall through */
+			fallthrough;
 		case 1:
 			priv->ep_tx_high_queue = 1;
 			priv->ep_tx_count++;