southbridge/intel: add IS_ENABLED() around Kconfig symbol references

Change-Id: I2b532522938123bb7844cef94cda0b44bcb98e45
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20350
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/southbridge/intel/common/spi.c

@@ -188,7 +188,7 @@ enum {
 	SPI_OPCODE_TYPE_WRITE_WITH_ADDRESS =	3
 };
 
-#if CONFIG_DEBUG_SPI_FLASH
+#if IS_ENABLED(CONFIG_DEBUG_SPI_FLASH)
 
 static u8 readb_(const void *addr)
 {