usb: dwc3: implement hw.usb.xhci.use_polling

Polling is currently only implemented in the xhci pci attachment.
Adding it to dwc3 doesn't make it much uglier, and supporting it can be
useful for confirming that hardware's otherwise functional when
interrupts are apparently not firing.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D38816