NTB: Fix static check warning in perf_clear_test

As pthr->dma_chan can't be NULL in this context, so there is
no need to check pthr->dma_chan.

Fixes: 99a06056 ("NTB: ntb_perf: Fix address err in perf_copy_chunk")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiasen Lin <linjiasen@hygon.cn>
Signed-off-by: Jon Mason <jdmason@kudzu.us>