ath9k: drop spectral packets after processing them

Spectral packets are "bogus" packets and should not be further evaluated
by the RX path.

Statistics are added to keep track of these packets.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>