mac80211: add key flag for management keys

Mark keys that might be used to receive management
frames so drivers can fall back on software crypto
for them if they don't support hardware offload.
As the new flag is only set correctly for RX keys
and the existing IEEE80211_KEY_FLAG_SW_MGMT flag
can only affect TX, also rename the latter to
IEEE80211_KEY_FLAG_SW_MGMT_TX.

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