NetworkPkg: Change OPTIONAL keyword usage style

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3760

Update all use of ', OPTIONAL' to ' OPTIONAL,' for function params.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>

NetworkPkg/UefiPxeBcDxe/PxeBcSupport.c

@@ -24,7 +24,7 @@
 EFI_STATUS
 PxeBcFlushStationIp (
   PXEBC_PRIVATE_DATA       *Private,
-  EFI_IP_ADDRESS           *StationIp,     OPTIONAL
+  EFI_IP_ADDRESS           *StationIp      OPTIONAL,
   EFI_IP_ADDRESS           *SubnetMask     OPTIONAL
   )
 {
@@ -1535,4 +1535,3 @@ CalcElapsedTime (
     Private->ElapsedTime = ElapsedTimeValue;
   }
 }
-