dwc_otg: make periodic scheduling behave properly for FS buses

If the root port is in full-speed mode, transfer times at 12mbit/s
would be calculated but matched against high-speed quotas.

Reinitialise hcd->frame_usecs[i] on each port enable event so that
full-speed bandwidth can be tracked sensibly.

Also, don't bother using the FIQ for transfers when in full-speed
mode - at the slower bus speed, interrupt frequency is reduced by
an order of magnitude.

Related issue: https://github.com/raspberrypi/linux/issues/2020