iwlwifi: Send broadcast probe request only when asked to

When running directed active scans we currently end up sending both the SSID
probe requests and an additional broadcast one.
This is due to the fact that we always leave the probe request template SSID IE
length to 0. Instead we should set it to the first SSID to scan, and fill the
direct_scan array with the remaining SSIDs to scan for. This way we only send
what we've been asked to: a broadcast probe request when no directed scan is
requested, and directed probe requests otherwise.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>