xhci: Init a transfers xhci, slotid and epid member on epctx alloc

xhci: Init a transfers xhci, slotid and epid member on epctx alloc

Transfers are part of an epctx, which is part of a slot, which is part of
a xhci. Transfers cannot dynamically be moved from one epctx to another,
so once created their xhci, slotid and epid are constant, so lets set these
up at creation time, rather then re-initializing them with the same
value each time a transfer gets submitted.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>