P2P: Update find_start timer only when p2p_scan is started.

P2P: Update find_start timer only when p2p_scan is started.

p2p->find_start timer was updated on each p2p_find call irrespective of
p2p_find being successful/failed/rejected. For cases where p2p_find was
in progress/pending, another call to p2p_find would be rejected but
p2p->find_start timer would still be updated.

p2p->find_start is maintained in wpa_supplicant to reject the kernel
scan entries before the p2p->find_start time. In above scenario, some of
the scan entries could be discarded even if the Probe Respons frame(s)
were received during the last scan/p2p_find.

This commit changes this to update the p2p->find_start timer only when
call to p2p_find is successful, i.e., a new scan is actually started.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>