xtensa: fix warning '"CONFIG_OF" is not defined'

The warning only shows up when building MMUv3 configuration with OF
support disabled.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/vectors.h

@@ -25,7 +25,7 @@
 #define XCHAL_KIO_DEFAULT_PADDR		0xf0000000
 #define XCHAL_KIO_SIZE			0x10000000
 
-#if XCHAL_HAVE_PTP_MMU && XCHAL_HAVE_SPANNING_WAY && CONFIG_OF
+#if XCHAL_HAVE_PTP_MMU && XCHAL_HAVE_SPANNING_WAY && defined(CONFIG_OF)
 #define XCHAL_KIO_PADDR			xtensa_get_kio_paddr()
 #else
 #define XCHAL_KIO_PADDR			XCHAL_KIO_DEFAULT_PADDR