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.

rtl8xxxu_core.c

@@ -1157,7 +1157,7 @@ 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);
@@ -1284,7 +1284,7 @@ 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;
@@ -1740,13 +1740,13 @@ static int rtl8xxxu_identify_chip(struct rtl8xxxu_priv *priv)
 	if (val16 & NORMAL_SIE_EP_TX_HIGH_MASK) {
 		priv->ep_tx_high_queue = 1;
 		priv->ep_tx_count++;
-		/* fall through */
+		fallthrough;
 	}
 
 	if (val16 & NORMAL_SIE_EP_TX_NORMAL_MASK) {
 		priv->ep_tx_normal_queue = 1;
 		priv->ep_tx_count++;
-		/* fall through */
+		fallthrough;
 	}
 
 	if (val16 & NORMAL_SIE_EP_TX_LOW_MASK) {
@@ -1763,11 +1763,11 @@ 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++;