staging: rtl8188eu: use safe iterator in rtw_free_xmitframe_queue

Use list_for_each_entry_safe, we may delete list items while iterating
over the list.

Fixes: 23017c88 ("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/20210517201826.25150-6-martin@kaiser.cx


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