SPI DMA Receive Request possibly wrong direction.

[stephenhensley]

In the SpiHandle::Impl::SetDmaPeripheral() function, the switch maps the hdma request to the corresponding DMA_REQUEST_SPIx_TX value regardless of direction.

I haven't done any SPI receive with DMA. So I can't say for sure that it won't work, but this definitely stood out as a potential issue.

We should investigate, and fix.

[TheSlowGrowth]

This may already be fixed with #472