Revert "spi: dw-dma: Get the last DMA scoop out of the FIFO"

This issue was due to a misconfiguration of the RP1 DMAC due to hardware
limitations, not the SPI driver (which was using the incorrect reported
maximum burst size to set the FIFO threshold).

This reverts commit 6aab06ff.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>