ath10k: make sure to use passive scan when n_ssids is 0

Normally user specifies broadcast ssid for
scanning. If the user wants to do a passive scan
it does not pass any ssids.

The patch makes sure we ath10k tells firmware to
not send anything at all in case it decides no
ssids equals broadcast ssid.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>