iwlwifi: mvm: remove variable shadowing

Variable "ac" defined twice. Fix that.

Fixes: commit 93f436e2 ("iwlwifi: mvm: set sta_id in SCD_QUEUE_CONFIG cmd")
Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>