iwlwifi: make U-APSD default configurable at compile time

With a significant number of deployed APs, enabling uAPSD leads to the
AP never using aggregation sessions (likely due to the complexities
involved in handling uAPSD in those.) This obviously results in a large
drop in throughput with such APs.

On the other hand, uAPSD can result in some power consumption benefits,
but for now just disable it to get performance with affected APs back
up.

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