iwlwifi: mvm: support enabling a queue with a given ssn

When enabling a queue, the default SSN is 0.

Allow determining what that SSN should be, if required. This
can happen, for example, if a queue gets reconfigured.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>