spi: sun6i: update max transfer size reported

The spi-sun6i driver have already got the ability to do large transfers.
However, the max transfer size reported is still fifo depth - 1.

Update the max transfer size reported to the max value possible.

Reported-by: Martin Ayotte <martinayotte@gmail.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Mark Brown <broonie@kernel.org>