Signal tx dma when spi fifo is less than half full,
and limit tx bursts to half the fifo length.

Clamp rx burst length to 1 to avoid alignment issues.
Signed-off-by: Huibin Hong <huibin.hong@rock-chips.com>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Mark Brown <broonie@kernel.org>