thunderbolt: Remove a meaningless NULL pointer check before dma_pool_destroy

dma_pool_destroy() already takes NULL pointer into account so there is
no need to check that again in tb_ctl_free().

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
[mw: reword commit log a bit]
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>