iwlwifi: mvm: Fix VIF specific debugfs directory creation

Avoid creating VIF specific debugfs directory if already exist.
This may happen when, for example, resetting hw, suspend-resume.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>