iwlwifi: fix fw version reading for DVM devices

In commit 97f95c93

 ("iwlwifi: remove support for fw older than
-16.ucode") we accidentally changed the fw version reading code for
DVM devices.  The code intended to remove the old fw version API,
because all MVM firmwares version 16 and above that we support don't
use it anymore.  But DVM devices still use the old FW API.

Fix that by bringing the code back in.

Reported-by: Pat Erley <pat-lkml@erley.org>
Tested-by: Kalle Valo <kvalo@codeaurora.org>
Fixes: 97f95c93

 ("iwlwifi: remove support for fw older than-16.ucode")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>