iwlwifi: mvm: take the transport ref back when leaving

If d0i3 is supported, we have released the initial transport reference
in iwl_op_mode_mvm_start(), so we should take it back in
iwl_op_mode_mvm_stop() to keep it balanced.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>