iwlwifi: s/txq_setup/txq_enable

iwlwifi: s/txq_setup/txq_enable

We need to be able to enable / disable Tx queues in HW
dynamically. So this function is no longer related to AGG
only. It can do the job for any queue, even AC ones. Change
the name to better reflect its role.

Also use the new function to configure the AC / CMD queues
in tx_start.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>