e1000: fix Tx hangs by disabling 64-bit DMA

Several users report issues with 32-bit adapters when plugged
into PCI slots in machines with >= 4GB ram.  In particular AMD
systems with HyperTransport to PCI bridges seem to trigger the
issue, but it isn't limited to only them.

This issue is not easily reproducible here, yet still continues
to occur in the field.  For e1000 on PCI devices, just disable DMA
addresses over the 4GB boundary when in PCI (not PCI-X) mode, to
prevent the issue from continuing to pop up.  The performance
impact for this is negligible.

The code was refactored to move the init of the hw struct to its
own function. This allows the init to be called very early in
probe, which then allows using hw-> members for this fix.

A slight refactor to the DMA mask code was done for minor
correctness based on the instructions in DMA-API-HOWTO.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>