wl12xx: mark no sched scan only after FW event

stop sched scan isn't an immediate operation
and we need to wait for PERIODIC_SCAN_COMPLETE_EVENT_ID
after sending a stop before changing internal state
and notifying upper layers.
Not doing this caused problems when canceling an existing sched
scan and immediately requesting to start a new one
with a different configuration as the FW was still
in the middle of the previous sched scan.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>