ath6kl: Fix off by one error in scan completion

When ath6kl was reworked to share code between regular and scheduled scans
in commit 3b8ffc6a ("ath6kl: Configure probed SSID list consistently"),
probed SSID entry changed from 1-index to 0-indexed. However,
ath6kl_cfg80211_scan_complete_event() was missed in that change. Fix its
indexing so that we correctly clear out the probed SSID list.

Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>