iwlwifi: use correct fw file in 8000 b-step

In 8000 B-step the FW file has changed, but by the time we
know the HW step, the FW file is already requested.

This patch defaults 8000 family to B-step if no HW step is
detected in time. When it can it checks what HW step it
really is (in 8000 family) and if it isn't B-step, the FW
file is released and the A-step file is requested.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>