iwlwifi: mvm: fix default disabled aggs in sta

For the ADD_STA command, when the flag for aggregation
disabling is set, there is a bitmap indicated what TIDs
are disabling aggregations and what aren't. Currently, by
default, all TIDs allow for aggregations since the value
we begin with is 0.

Change this default value to 0xffff so all TIDs don't
allow aggregations until explicitly turned on.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>