iwlwifi: wipe out the status of the SCD when we disable a queue

When we disable a queue, we don't want the SCD to remember anything
about this queue (what packet was transmitted but not acked, what
packed was acked etc...).
Wipe out all this data in its SRAM.

Constify the arguments to iwl_write_targ_mem_dwords on the way.

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