carl9170: fix signal strength reporting issues

On A-MPDU frames, the hardware only reports valid
signal strength data for the last subframe.

This patch fixes it by flagging everything but the
last subframe in an A-MPDU to tell mac80211 to
ignore the signal strength entirely. Otherwise
the empty value (= 0 dbm) will distort the
average quite badly.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>