staging: r8188eu: make rtw_handle_dualmac a void function

The rtw_handle_dualmac function always returns _SUCCESS. Remove the return
value and update the one caller that checks it.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220730143939.671951-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>