mac80211: add an rx flag for ignoring a packet's signal strength

For A-MPDU rx it makes sense to only process the signal strength once per
aggregate instead of once per subframe. Additonally, some hardware (e.g.
Atheros) only provides valid signal strength information for the last
subframe.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>