Merge pull request #390 from P33M/rpi-3.6.y

dwc_otg: Don't issue traffic to LS devices in FS mode
dwc_otg: Implement trans_backoff paramater hack

This adds a configurable, disabled by default parameter that prevents
queueing of non-periodic packets if there are less than trans_backoff
PHY core clocks remaining in the microframe. This is noted to assist
isochronous performance with high bandwidths - see #82.

Caution: Do not set this value close to or above the number of PHY
core clocks in a microframe (6000). Doing so will prevent any non-
periodic transactions from happening.

Thanks to @KrfVan for the original experimentation work.