ath6kl: Use correct max-scan-SSIDs limit

The currently used firmware images support 16 SSIDs in the scan
request (indexes 0..15), so update the host driver to use the same
limit to allow some more SSIDs to be scanned per request. In addition,
change the max-index to max-SSIDs to make it easier to understand the
implementation and fix couple of off-by-one checks that could limit
the maximum number of entries too strictly.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>