southbridge/amd/agesa/hudson/early_setup.c: Use IS_ENABLED macro

Change-Id: I2adb5a8fe2cede988cc6fdef5ff81da86d267175
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7624
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>