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: NMartin Ayotte <martinayotte@gmail.com>
Signed-off-by: NIcenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: NMark Brown <broonie@kernel.org>