mac80211_hwsim: add hwsim_tx_rate_flags to netlink attributes

For correct interpretation of a tx rate, the corresponding rate flags are
needed (e.g. whether a HT-MCS rate or a legacy rate) and moreover for more
correct simulation the other infos of the flags are important (like
short-GI). Keeping compatibility, the flags are not integrated into the
existing hwsim_tx_rate, but transmitted as an additional netlink attribute.

Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>