Fix following problems about VLAN driver:

1. "VLAN Configuration” form crash after 'reconnect -r'
2. disconnect VLAN driver from the managed device will return EFI_NOT_FOUND
3. disconnect MNP driver from the managed device, VLAN driver couldn't be stopped.
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ni, Ruiyu <ruiyu.ni@intel.com>
Reviewed-by: Gao, Liming <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15174 6f19259b-4bc3-4df7-8a09-765794883524