Nl80211 optimisation + retrieve connection speed through nl80211 in iwd

There are three commits that change the nl80211 commands used by:

nm_platform_wifi_get_frequency
nm_platform_wifi_get_quality
nm_platform_wifi_get_rate
nm_platform_wifi_get_bssid

This should hopefully not affect the wpa_supplicant backend except reducing the cycles used a bit. nm-device-wifi.c avoids updating quality/rate when a scan is running. These checks could probably be removed now although I guess even without these patches when NL80211_CMD_GET_SCAN is used they should have worked fine, so I'm not sure why the check is there. It's true that the bssid/quality/rates can't change during a scan if the scan is fully occupying the network interface..

This branch is based on the iwd-hidden-networks-cleanup branch (merge request !646 (closed)) because the patches don't apply directly to master, I'll rebase this branch if the other branch is merged.

On a different note wifi_nl80211_get_rate() returns the Tx rate. I wonder if rather Rx is what a user expects when they see just one number.