Stop using usb_{alloc,free}mem() for the rings and internal structures.

Stop using usb_{alloc,free}mem() for the rings and internal structures.

Since xhci(4) does not allocate memory for its rings in interrupt
context, it has no use for the free lists offered by the USB memory
allocator.

Using bus_dmamem_alloc(9) and friends also allows us to respect the
boundary requirement for the various structures specified in Table 54.

While here make use of defines for every alignment and boundary
requirements which are different than a page size.