iwlwifi: pcie: refrain from SCD accesses

Up till now we accessed SCD configuration only for initial
configuration and for enabling command queue.
For a000 generation the command queue is open by default
and firmware configures the rest. No driver SCD accesses
are expected. Make sure this is the case.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>