iwlwifi: mvm: Re-factor enabling uAPSD logic

The driver can enable uAPSD and specify some of its related parameters.
This patch organizes this logic in a separate function.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Reviewed-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>