staging: rtl8723au: fix sparse warning

drivers/staging/rtl8723au/core/rtw_xmit.c:2375 warning: symbol
'rtw_ack_tx_done23a' was not declared. Should it be static?

Function 'rtw_ack_tx_done23a' seems to be unused in current staging
code.
Signed-off-by: Alexey Tulia <alexey.tulia@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723au/core/rtw_xmit.c

@@ -2372,12 +2372,3 @@ int rtw_ack_tx_wait23a(struct xmit_priv *pxmitpriv, u32 timeout_ms)
 	return rtw_sctx_wait23a(pack_tx_ops);
 }
 
-void rtw_ack_tx_done23a(struct xmit_priv *pxmitpriv, int status)
-{
-	struct submit_ctx *pack_tx_ops = &pxmitpriv->ack_tx_ops;
-
-	if (pxmitpriv->ack_tx)
-		rtw23a_sctx_done_err(&pack_tx_ops, status);
-	else
-		DBG_8723A("%s ack_tx not set\n", __func__);
-}