MdeModulePkg:Fix a robustness issue of Mnp Driver

MdeModulePkg:Fix a robustness issue of Mnp Driver

v3:
*
When there exists duplicate items in VLAN variable , save the correct
variable content back to the variable storage after duplicate items are removed

Duplicate items in VLAN variable will cause MNP driver binding
start function fall into infinite loop,so we should check it's
content before using it.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>