iwlwifi: mvm: fix bug in scan ssid

Increment index in each iteration. Without this increment we are
overriding the added SSIDs and we will send only the last SSId
and (n_ssids - 1) broadcast probes.

Cc: <stable@vger.kernel.org> [3.9+]
Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>