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

drivers/spi/spi-sun6i.c

@@ -194,9 +194,7 @@ static void sun6i_spi_set_cs(struct spi_device *spi, bool enable)
 
 static size_t sun6i_spi_max_transfer_size(struct spi_device *spi)
 {
-	struct sun6i_spi *sspi = spi_master_get_devdata(spi->master);
-
-	return sspi->fifo_depth - 1;
+	return SUN6I_MAX_XFER_SIZE - 1;
 }
 
 static int sun6i_spi_transfer_one(struct spi_master *master,