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>