loader/powerpc64: Fix HV check for CAS usage

Logic was backwards.  The function returns true if it *is* running as a
hypervisor, whereas we want to only call the CAS utility if we're running as a
guest.

Reported by:	Shawn Anastasio <shawn@anastas.io>

stand/powerpc/ofw/cas.c

@@ -203,7 +203,7 @@ ppc64_cas(void)
 	}
 
 	/* Skip CAS when running on PowerNV */
-	if (!ppc64_hv())
+	if (ppc64_hv())
 		return (0);
 
 	ihandle = OF_open("/");