l2_packet_ndis: Fix overlapped write not to corrupt stack

l2_packet_ndis: Fix overlapped write not to corrupt stack

When using overlapped write, we must have the provided memory
areas available during the operation and cannot just use stack
unless we wait for the completion within the function. In the case
of TX here, we can easily wait for the completion since it is likely
to happen immediately. In addition, this provides more reliable
success/failure return value for l2_packet_send(). [Bug 328]