staging: r8188eu: use kernel helper to iterate over a list

rtw_free_xmitframe_list iterates over the list of xmit_frames and frees
each entry. We can use list_for_each_entry_safe instead of coding this
manually. We need the _safe version as the current pxmitframe will be
removed from the list by rtw_free_xmitframe.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20230130195303.138941-6-martin@kaiser.cx


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