nl80211: add support for SAE authentication offload

Let drivers advertise support for station-mode SAE authentication
offload with a new NL80211_EXT_FEATURE_SAE_OFFLOAD flag.

Signed-off-by: Chung-Hsien Hsu <stanley.hsu@cypress.com>
Signed-off-by: Chi-Hsien Lin <chi-hsien.lin@cypress.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>