Use PIO USB driver instead of DMA

[aadavids]

I was getting some network performance issues when using a USB cellular modem. The modem worked fine most of the time, but when downloading files in the 20+MB range, the download slowed to a crawl, repeatedly timed out. While the download was being attempted, other network traffic (pings, etc) couldn't get through.

Changing the DMA driver to the PIO USB driver resolved this issue. In addition, the Beaglebone Black debian config uses the PIO driver, so best to match their config as much as possible.

[fhunleth]

Thanks! I tested this out. It appears to lower throughput on the USB device I was using (a WiFi module) by 30-50%, but that would seem expected for turning on DMA. Given that this is the Debian configuration, it works so much better for your situation, and RCN on the Beagleboard mailing list seems to indicate that there are compatibility issues with DMA mode (See thread), I think we should default to this configuration and let users switch to DMA if they need the throughput.