xHCI: Allocate 2 segments for transfer ring

Allocate 2 segments for transfer ring by default, so we can expand the ring
when the enqueue pointer and dequeue pointer are in different segments.

Signed-off-by: Andiry Xu <andiry.xu@amd.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Tested-by: Paul Zimmerman <Paul.Zimmerman@synopsys.com>