Ignore scan results from ongoing scan when FLUSH command is issued

Ignore scan results from ongoing scan when FLUSH command is issued

This makes wpa_supplicant behavior more consistent with FLUSH command to
clear all state. Previously, it was possible for an ongoing scan to be
aborted when the FLUSH command is issued and the scan results from that
aborted scan would still be processed and that would update the BSS
table which was supposed to cleared by the FLUSH command.

This could result in hwsim test case failures due to unexpected BSS
table entries being present after the FLUSH command.

Signed-off-by: Jouni Malinen <j@w1.fi>