nl80211: add an option to allow MFP without requiring it

The user space can now allow the kernel to associate to an AP that
requires MFP or that doesn't have MFP enabled in the same
NL80211_CMD_CONNECT command, by using a new NL80211_MFP_OPTIONAL flag.
The driver / firmware will decide whether to use it or not.

Include a feature bit to advertise support for NL80211_MFP_OPTIONAL.
This allows new user space to run on old kernels and know that it
cannot use the new attribute if it isn't supported.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>