staging: r8188eu: simplify the sta loop in rtw_dequeue_xframe

staging: r8188eu: simplify the sta loop in rtw_dequeue_xframe

Use list_for_each_entry_safe to iterate over the station entries in
function rtw_dequeue_xframe instead of coding the loop manually.

We have to use the safe version, the loop body may remove a station from
the list over which we iterate.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230123205342.229589-6-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>