media: stm32: dcmi: fix some error handling bugs in probe()

In the Linux kernel, the following vulnerability has been resolved:

media: stm32: dcmi: fix some error handling bugs in probe()

There are a few issues here:

1) After we assign:
chan = dma_request_chan(&pdev->dev, "tx");
Then the error paths need to clean up before returning. The first
error path does a direct return.
2) The error paths check "dcmi->mdma_chan" but that is not assigned
until later so it results in memory leaks. Test "mdma_chan"
instead.
3) The error handling calls dma_release_channel(dcmi->dma_chan) before
"dcmi->dma_chan" has been assigned which leads to a NULL pointer
dereference. Use the "chan" variable instead.

I also moved the call to dma_release_channel() after the call to
dma_release_channel() so it mirrors the allocation code better.