PCI: endpoint: Remove surplus return statement from pci_epf_test_clean_dma_chan()

PCI: endpoint: Remove surplus return statement from pci_epf_test_clean_dma_chan()

Remove a surplus return statement from the void function that has been
added in the commit commit 8353813c88ef ("PCI: endpoint: Enable DMA
tests for endpoints with DMA capabilities").

Especially, as an empty return statements at the end of a void functions
serve little purpose.

This fixes the following checkpatch.pl script warning:

  WARNING: void function return statements are not generally useful
  #296: FILE: drivers/pci/endpoint/functions/pci-epf-test.c:296:
  +     return;
  +}

Link: https://lore.kernel.org/r/tencent_F250BEE2A65745A524E2EFE70CF615CA8F06@qq.com
Signed-off-by: Wang Jiang <jiangwang@kylinos.cn>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>