staging: rtl8188eu: use safe iterator in rtw_free_all_stainfo

staging: rtl8188eu: use safe iterator in rtw_free_all_stainfo

This is another case where we may remove list entries while we iterate over
the list. Use list_for_each_entry_safe to avoid an endless loop.

Fixes: 23017c8842d2 ("staging: rtl8188eu: Use list iterators and helpers")
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210517201826.25150-2-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>