Default to disabling PCI DAC as this option appears unsafe on amd64
(original suggestion by Hans-Frieder Vogt <hfvogt@arcor.de>).

The driver will typically report PCI System error when something goes
wrong. The relevant interrupt is not masked any more and the driver
can thus be disabled.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>