spi: sun4i: update max transfer size reported

spi: sun4i: update max transfer size reported

The spi-sun4i driver already has 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.

Fixes: 196737912da5 ("spi: sun4i: Allow transfers larger than FIFO size")
Signed-off-by: Jonathan Liu <net147@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20200727072328.510798-1-net147@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>