software fixes

[sergachev]

• dev_dbg -> dev_err for DMA overflows and underflows
• write all patterns before starting verifying them in dma_test
• zero read buffers in dma_test for better error detection
• minor edits

[sergachev]

Just noticed, it addresses the same as https://github.com/enjoy-digital/litepcie/pull/72, but it's better to make the variable local. Guess this can be merged on top.

[enjoy-digital]

Thanks @sergachev, this is merged.