wpaspy: Handle DETACH response more robustly

wpaspy: Handle DETACH response more robustly

There could be pending unsolicited event messages on the monitor socket
when the DETACH command is issued. As such, the response may be
something else then OK even if the actual detach operation succeeded.
Try to avoid this be dropping pending messages before issuing the detach
command. As an additional workaround, check the response against FAIL
instead of requiring OK so that the self.attached does not get left to
True incorrectly even if an additional event message were to be
received.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>