mac80211: fix CTS protection handling

The rates[0] CTS and RTS flags are only set after rate control has been
called, so minstrel cannot use them to for setting the number of
retries. This patch adds two new flags to explicitly indicate RTS/CTS use.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>