Make function rtw_sta_flush to void in order to
prevent cocci warning as follows:

  Unneeded variable: "ret". Return "0" on line 1031

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Kang Minchul <tegongkang@gmail.com>
Link: https://lore.kernel.org/r/20221025170621.271903-5-tegongkang@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>