Enable the read DMA engine's PCI read request burst length long burst

Enable the read DMA engine's PCI read request burst length long burst
mode (4KB) for PCIe chips. This resolves the poor TX performance for
the PCIe chips. The result being a bit under double the TX performance
on a Gig connection (roughly 495 Mb/s -> 940 Mb/s).

Tested by reyk@, sthen@, brad@ and a few end users.