mac80211: add IEEE80211_KEY_FLAG_GENERATE_MMIE to ieee80211_key_flags

Add IEEE80211_KEY_FLAG_GENERATE_MMIE flag to ieee80211_key_flags in order
to allow the driver to notify mac80211 to generate MMIE and that it
requires sequence number generation only.
This is a preliminary patch to add BIP_CMAC_128 hw support to mt7615
driver

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/dfe275f9aa0f1cc6b33085f9efd5d8447f68ad13.1563228405.git.lorenzo@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>