spi: img-spfi: Increase DMA burst size

A 1-byte burst size is rather inefficient and has been shown to cause
TX issues during testing.  Increase the DMA burst size to 4-bytes for
both RX and TX DMA when using the 8-bit FIFO.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>