spi: sun6i: reduce DMA RX transfer width to single byte

In the Linux kernel, the following vulnerability has been resolved:

spi: sun6i: reduce DMA RX transfer width to single byte

Through empirical testing it has been determined that sometimes RX SPI

transfers with DMA enabled return corrupted data. This is down to single

or even multiple bytes lost during DMA transfer from SPI peripheral to

memory. It seems the RX FIFO within the SPI peripheral can become

confused when performing bus read accesses wider than a single byte to it

during an active SPI transfer.

This patch reduces the width of individual DMA read accesses to the

RX FIFO to a single byte to mitigate that issue.