spi: dw: revisit FIFO size detection again

The commit d297933cc7 (spi: dw: Fix detecting FIFO depth) tries to fix the
logic of the FIFO detection based on the description on the comments. However,
there is a slight difference between numbers in TX Level and TX FIFO size.

So, by specification the FIFO size would be in a range 2-256 bytes. From TX
Level prospective it means we can set threshold in the range 0-(FIFO size - 1)
bytes. Hence there are currently two issues:
  a) FIFO size 2 bytes is actually skipped since TX Level is 1 bit and could be
     either 0 or 1 byte;
  b) FIFO size is incorrectly decreased by 1 which already done by meaning of
     TX Level register.

This patch fixes it eventually right.

Fixes: d297933cc7 (spi: dw: Fix detecting FIFO depth)
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

drivers/spi/spi-dw.c

@@ -621,14 +621,14 @@ static void spi_hw_init(struct device *dev, struct dw_spi *dws)
 	if (!dws->fifo_len) {
 		u32 fifo;
 
-		for (fifo = 2; fifo <= 256; fifo++) {
+		for (fifo = 1; fifo < 256; fifo++) {
 			dw_writew(dws, DW_SPI_TXFLTR, fifo);
 			if (fifo != dw_readw(dws, DW_SPI_TXFLTR))
 				break;
 		}
 		dw_writew(dws, DW_SPI_TXFLTR, 0);
 
-		dws->fifo_len = (fifo == 2) ? 0 : fifo - 1;
+		dws->fifo_len = (fifo == 1) ? 0 : fifo;
 		dev_dbg(dev, "Detected FIFO size: %u bytes\n", dws->fifo_len);
 	}
 }