iwlwifi: mvm: add support for NICs which have only 16 Tx queues.

Some NICs embedded in platforms that have only 16 Tx queues,
this affect the mapping of the Tx queues.

Signed-off-by: Eytan Lifshitz <eytan.lifshitz@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>