xtensa: nommu: fix USER_RING definition

There's no kernel/user separation in noMMU and PS.RING may not exist.
Even if it exists it should not be used because TLB entries are not set
up for user ring on user pages.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/pgtable.h

@@ -18,7 +18,11 @@
  * We only use two ring levels, user and kernel space.
  */
 
+#ifdef CONFIG_MMU
 #define USER_RING		1	/* user ring level */
+#else
+#define USER_RING		0
+#endif
 #define KERNEL_RING		0	/* kernel ring level */
 
 /*