spi: img-spfi: Select FIFO based on transfer length

Since the 32-bit FIFO is deeper (64 bytes) than the 8-bit FIFO (16 bytes),
use the 32-bit FIFO when there are at least 32 bits remaining to be
transferred in PIO mode or when the transfer length is 32-bit aligned
in DMA mode.

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