iwlwifi: mvm: prepare for scheduler config command

The scheduler is a HW sub-block that directs the work of the Flow
Handler by issuing requests for frame transfers, specifying source
and destination. Its primary function is to allocate flows into the
TX FIFOs based upon a pre-determined mapping.

The driver has some responsibilities to the scheduler, namely
initialising and maintaining the hardware registers. This is
currently done by directly accessing them, which can cause races
with the firmware also accessing the registers.

To address this problem, change the driver to no longer directly
access the registers but go through the firmware for this if the
firmware has support for DQA and thus the new command.

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