ar9170: Add support for D-Link DWA 160 A2

At least two revisions of the D-Link DWA 160 exist, called A1 and A2. A1
(USB-ID 07d1:3c10) is already listed in usb.c as D-Link DWA 160A. A2
(USB-ID 07d1:3a09) works if added to ar9170_usb_ids. I didn't do much
testing until now, but I was able to connect to APs using WPA or WEP and
transmit data.

Summary:

* Add model revision number to the comment for D-Link DWA 160 A1 (07d1:3c10)
* Add support for D-Link DWA 160 A2 (07d1:3a09)

Signed-off-by: Thomas Klute <thomas2.klute@uni-dortmund.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>