ath9k: correctly handle short radar pulses

ath9k: correctly handle short radar pulses

In commit e4673cb42eaa ("ath9k: discard undersized packets")
the lower bound of RX packets was set to 10 (min ACK size) to
filter those that would otherwise be treated as invalid at
mac80211.

Alas, short radar pulses are reported as PHY_ERROR frames
with length set to 3. Therefore their detection stopped
working after that commit.

NOTE: ath9k drivers built thereafter will not pass DFS
certification.

This extends the criteria for short packets to explicitly
handle PHY_ERROR frames.

Fixes: e4673cb42eaa ("ath9k: discard undersized packets")
Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>