[realtek] Enable DAC only when built as a 64-bit binary

[realtek] Enable DAC only when built as a 64-bit binary

Some RTL8169 cards (observed with an RTL8169SC) crash and burn if DAC
is enabled, even if only 32-bit addresses are used.  Observed
behaviour includes system lockups and repeated transmission of garbage
data onto the wire.

This seems to be a known problem.  The Linux r8169 driver disables DAC
by default and provides a "use_dac" module parameter.

There appears to be no known test for determining whether or not DAC
will work.  As a workaround, enable DAC only if we are built as as
64-bit binary.  This at least eliminates the problem in the common
case of a 32-bit build, which will never use 64-bit addresses anyway.

Signed-off-by: Michael Brown <mcb30@ipxe.org>