dwmmc: Multiple busdma fixes.

• limit maximum segment size to 2048 bytes. Although dwmmc supports a buffer fragment with a maximum length of 4095 bytes, use the nearest lower power of two as the maximum fragment size. Otherwise, busdma create excessive buffer fragments.
• fix off by one error in computation of the maximum data transfer length.
• in addition, reserve two DMA descriptors that can be used by busdma bouncing. The beginning or end of the buffer can be misaligned.
• Don’t ignore errors passed to bus_dmamap_load() callback function.
• In theory, a DMA engine may be running at time when next dma descriptor is constructed. Create a full DMA descriptor before OWN bit is set.

MFC after: 2 weeks