XMC4800 Ethernet DMA descriptor selection

I tested with the Normal DMA descriptor layout, but that didn't work. Changing to Alternate 16-bytes layout worked. It appears as the Alternate 16-bytes descriptors are the default. The reference manual should be updated to reflect that.

Also, if Normal descriptors are not implemented in hardware, as it seems to be the case, then the section in the reference manual covering Normal descriptor should be removed.