EmbeddedPkg: 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: Andrew Fish <afish@apple.com>

EmbeddedPkg/Library/PrePiLib/FwVol.c

@@ -156,7 +156,7 @@ FileHandleToVolume (
 EFI_STATUS
 FindFileEx (
   IN  CONST EFI_PEI_FV_HANDLE        FvHandle,
-  IN  CONST EFI_GUID                 *FileName,   OPTIONAL
+  IN  CONST EFI_GUID                 *FileName    OPTIONAL,
   IN        EFI_FV_FILETYPE          SearchType,
   IN OUT    EFI_PEI_FILE_HANDLE      *FileHandle
   )