dwc_otg: Don't issue traffic to LS devices in FS mode

Issuing low-speed packets when the root port is in full-speed mode
causes the root port to stop responding. Explicitly fail when
enqueuing URBs to a LS endpoint on a FS bus.
Merge pull request #390 from P33M/rpi-3.6.y

dwc_otg: Don't issue traffic to LS devices in FS mode