xHCI: refine td allocation

In xhci_urb_enqueue(), allocate a block of memory for all the TDs instead
of allocating memory for each of them separately. This reduces the number
of kzalloc calling when an isochronous usb is submitted.

Signed-off-by: Andiry Xu <andiry.xu@amd.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>