MdeModulePkg/DxeNetLib: Fix a potential issue in macro definition.

MdeModulePkg/DxeNetLib: Fix a potential issue in macro definition.

The NIC_ITEM_CONFIG_SIZE macro in DxeNetLib is defined as:
sizeof (NIC_IP4_CONFIG_INFO) + sizeof (EFI_IP4_ROUTE_TABLE) *
MAX_IP4_CONFIG_IN_VARIABLE. This macro should be surrounded
with parenthesis to avoid being incorrectly used.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wang Fan <fan.wang@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>