ath9k: Check virtual wiphy state on tx()

mac80211 should not be requesting us to transmit frames on paused
wiphys since we stop the TX queues. Just in case, add debug code to
make sure we catch if this were to happen.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>