mac80211: propagate the support for TWT to the driver

TWT is a feature that was added in 11ah and enhanced in
11ax. There are two bits that need to be set if we want
to use the feature in 11ax: one in the HE Capability IE
and one in the Extended Capability IE. This is because
of backward compatibility between 11ah and 11ax.

In order to simplify the flow for the low level driver
in managed mode, aggregate the two bits and add a boolean
that tells whether TWT is supported or not, but only if
11ax is 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>