spi: sun6i: Allow transfers larger than FIFO size

The spi-sun6i driver have the same problem that spi-sun4i used to have
-- SPI transfers are limited to one FIFO depth.

This commit fixes this problem in the same way it's fixed in spi-sun4i.
See commit 19673791 ("spi: sun4i: Allow transfers larger than FIFO size")
for more information.

The sun6i SPI controllers features changeable interrupt trigger level, but I
set it to 3/4 of fifo depth, as same as the the sun4i SPI controllers.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>