ath10k: allow empty ssid vdev config

It doesn't make much sense to reject a valid
firmware configuration combination.

Since SSID isn't known early on it might make
sense to allow driver to start vdev without SSID
and restart it later.

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