staging: rtl8192e: 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>
Link: https://lore.kernel.org/r/20200727191006.GA30474@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8192e/rtllib_wx.c

@@ -680,7 +680,7 @@ int rtllib_wx_set_mlme(struct rtllib_device *ieee,
 	switch (mlme->cmd) {
 	case IW_MLME_DEAUTH:
 		deauth = true;
-		/* fall through */
+		fallthrough;
 	case IW_MLME_DISASSOC:
 		if (deauth)
 			netdev_info(ieee->dev, "disauth packet !\n");