ShellPkg: refine the logic for cp command

Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>
Reviewed-by: Erik Bjorge <Erik.c.Bjorge@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14394 6f19259b-4bc3-4df7-8a09-765794883524