staging: rtl8723bs: 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/20200727190657.GA30194@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723bs/core/rtw_mlme_ext.c

@@ -568,7 +568,7 @@ void mgt_dispatcher(struct adapter *padapter, union recv_frame *precv_frame)
 			ptable->func = &OnAuth;
 		else
 			ptable->func = &OnAuthClient;
-		/* fall through */
+		fallthrough;
 	case WIFI_ASSOCREQ:
 	case WIFI_REASSOCREQ:
 		_mgt_dispatcher(padapter, ptable, precv_frame);