MdeModulePkg: Add new macros and refine codes

Add 2 macros inNetLib.h
#define  IP4_MASK_MAX          32
#define  IP6_PREFIX_MAX        128
we will use these two macros to check the max mask/prefix length,
instead of
#define  IP4_MASK_NUM          33
#define  IP6_PREFIX_NUM        129
which means a valid number.
This will make the code readability and maintainability.

Cc: Subramanian Sriram <sriram-s@hpe.com>
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: Sriram Subramanian <sriram-s@hpe.com>

MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Impl.c

@@ -838,7 +838,7 @@ Ip4StationAddressValid (
   //
   // Only support the continuous net masks
   //
-  if ((Len = NetGetMaskLength (Netmask)) == IP4_MASK_NUM) {
+  if ((Len = NetGetMaskLength (Netmask)) == (IP4_MASK_MAX + 1)) {
     return FALSE;
   }