ath9k_htc: Set proper firmware offset for Netgear WNDA3200

Netgear WNDA3200 device uses ar7010 firmware but it is failed to set
correct firmware offset on firmware download which causes device initialization
failure.

Cc: stable@kernel.org
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>