iwlwifi: mvm: disable uAPSD due to bugs in the firmware

The current firmware advertises support for uAPSD, but
critical bugs force us to disable the feature.
When a fixed firmware will be available, we will be able to
re-enable uAPSD.

Cc: <stable@vger.kernel.org> [3.13+]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>