nl80211: Use the new NL80211_MFP_OPTIONAL option

nl80211: Use the new NL80211_MFP_OPTIONAL option

Now we can configure the network block so that it allows MFP setting for
the NL80211_CMD_CONNECT command. If the kernel finds an AP that requires
MFP, it'll be able to connect to it.

Note that since NL80211_MFP_OPTIONAL isn't supported for
NL80211_CMD_ASSOCIATE, we need to take the MFP configuration outside
nl80211_connect_common(). In addition, check that
NL80211_EXT_FEATURE_MFP_OPTIONAL is supported, to be backward compatible
with older kernels.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>