thunderbolt: Use kmemdup instead of kzalloc and memcpy

Fixes coccicheck warning:
drivers/thunderbolt/dma_test.c:302:13-20: WARNING opportunity for kmemdup.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>