PCI: endpoint: pci-epf-test: Set dma_chan_rx pointer to NULL on error

PCI: endpoint: pci-epf-test: Set dma_chan_rx pointer to NULL on error

If dma_chan_tx allocation fails, set dma_chan_rx to NULL after it is
freed.

Link: https://lore.kernel.org/r/20241227160841.92382-1-khalfella@gmail.com
Fixes: 8353813c88ef ("PCI: endpoint: Enable DMA tests for endpoints with DMA capabilities")
Signed-off-by: Mohamed Khalfella <khalfella@gmail.com>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>