The current logic around the 'use_dac' module parameter prevents the
r81969 driver from being loadable on 64-bit systems without any RAM
below 4 GB when the parameter is left at its default value.

So introduce a new default value -1 which indicates that 64-bit DMA
should be enabled on sufficiently recent PCIe chips, i.e., versions
RTL_GIGA_MAC_VER_18 or later. Explicit param values of 0 or 1 retain
the existing behavior of unconditionally enabling/disabling 64-bit DMA
on 64-bit architectures (i.e., regardless of the type and version of the
chip)

Since PCIe chips do not need to CPlusCmd Dual Address Cycle to be set,
make that conditional on the device type as well.

Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>