Instead of keeping a "frame list" like older host controllers, the xHCI
host controller keeps internal representations of the USB devices, with a
transfer ring per endpoint.  The host controller queues Transfer Request
Blocks (TRBs) to the endpoint ring, and then "rings the doorbell" for that
device.  The host controller processes the transfer, places a transfer
completion event on the event ring, and interrupts the system.

The device context base address array must be allocated by the xHCI host
controller driver, along with the device contexts it points to.
Signed-off-by: NSarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>