ath6kl: Only use match sets when firmware supports it

ath6kl: Only use match sets when firmware supports it

Commit dd45b7598f1c ("ath6kl: Include match ssid list in scheduled scan")
merged the probed and matched SSID lists before sending them to the
firmware. In the process, it assumed match set support is always available
in ath6kl_set_probed_ssids, which breaks scans for hidden SSIDs. Now, check
that the firmware supports matching SSIDs in scheduled scans before setting
MATCH_SSID_FLAG.

Fixes: dd45b7598f1c ("ath6kl: Include match ssid list in scheduled scan")
Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>