ath10k: set max_num_vdevs based on wmi op version

To make it easier to manage firmware differences, we should not use
ATH10K_FW_FEATURE_WMI_10X outside ath10k_core_init_firmware_features(). To
achieve that create new field ar->max_num_vdevs and set it based on wmi op
version.

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