nl80211: Allow set/del pmksa operations for AP

Host drivers may offload authentication to the user space
through the commit ("cfg80211: Authentication offload to
user space in AP mode").

This interface can be used to implement SAE by having the
userspace do authentication/PMKID key derivation and driver
handle the association.

A step ahead, this interface can get further optimized if the
PMKID is passed to the host driver and also have it respond to
the association request by the STA on a valid PMKID.

This commit enables the userspace to pass the PMKID to the host
drivers through the set/del pmksa operations in AP mode.

Set/Del pmksa is now restricted to STA/P2P client mode only and
thus the drivers might not expect them in any other(AP) mode.

This commit also introduces a feature flag
NL80211_EXT_FEATURE_AP_PMKSA_CACHING (johannes: renamed) to
maintain the backward compatibility of such an expectation by
the host drivers. These operations are allowed in AP mode only
when the drivers advertize the capability through this flag.

Signed-off-by: Liangwei Dong <liangwei@codeaurora.org>
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>
[rename flag to NL80211_EXT_FEATURE_AP_PMKSA_CACHING]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>