iwlwifi: s/txq_agg_disable/txq_disable

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.

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