[zircon][xhci] Add complete_cb as a param to hci_request_queue op

usb_request_t now has public and private portions for each driver in usb
stack. The complete_cb is different for each layer. At the moment we are
using the same req->complete_cb and modifying it when the request passes
down the stack. By making a private field for complete_cb in each
driver, we make the code less error prone. This changeset modifies the
request_queue hci protocol op to take complete_cb as a parameter. The
hci layer saves the callback in its private context and uses the one in
the private context when the request is complete. After all the usage
of complete_cb in usb_request_t are removed, I wll remove the
unnecessary fields.

Test: Vim2: fx serve. iochk on usb mass storage device.
Astro: netboot. lsusb. Verified usb network adapter showed up
Pixelbook: fx serve using usb network adapter. iochk on usb mass storage
device. audio -d 1 tone on usb audio device.

Change-Id: Ia5bb083325a9ec2cb7547a378b58a9cbb1e60824