ath9k_htc: add AVM FRITZ!WLAN 11N v2 support

This was reported and tested by Martin Walter over at AVM GmbH Berlin.
This also applies to 3.0.1 so sendint to stable.

Cc: s.kirste@avm.de
Cc: d.friedel@avm.de
Cc: Martin Walter <m.walter@avm.de>
Cc: Peter Grabienski <pgrabien@qca.qualcomm.com>
Cc: stable@kernel.org
Tested-by: Martin Walter <m.walter@avm.de>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>