P2P: Fix scan_res_handler setting for p2p_scan

P2P: Fix scan_res_handler setting for p2p_scan

The previous version set scan_res_handler unconditionally and then
cleared it if scan request failed. This can result in incorrect clearing
of the handler to NULL for a previously started scan that has not yet
completed. This can make p2p_find command fail to use the
start-after-scan-completion mechanism in some cases. Fix this by setting
scan_res_handler properly after having verified that the driver command
for starting the scan was successful.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>
intended-for: hostap-1