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

Change-Id: I8fabb7331435eb518a5c95cb29c4ff5ca98560d2
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20349
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/southbridge/amd/agesa/hudson/spi.c

@@ -110,7 +110,7 @@ static int spi_ctrlr_xfer(const struct spi_slave *slave, const void *dout,
 
 	readoffby1 = bytesout ? 0 : 1;
 
-#if CONFIG_SOUTHBRIDGE_AMD_AGESA_YANGTZE
+#if IS_ENABLED(CONFIG_SOUTHBRIDGE_AMD_AGESA_YANGTZE)
 	spi_write(0x1E, 5);
 	spi_write(0x1F, bytesout); /* SpiExtRegIndx [5] - TxByteCount */
 	spi_write(0x1E, 6);