mac80211_hwsim: enable IEEE80211_HW_SUPPORT_FAST_XMIT

For hwsim, the duration field in frames is already not valid for
the common case of HT/VHT MCSes, so there's little point in trying
to keep it accurate for the legacy rates. Enable the fast-xmit code
to allow testing that, although given the dependency on hardware
crypto it will only be enabled in open network configurations.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>