spi: spi-loopback-test: Fix 'tx_buf' might be 'rx_buf'

In function 'spi_test_run_iter': Value 'tx_buf' might be 'rx_buf'.
Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Link: https://lore.kernel.org/r/1620629903-15493-5-git-send-email-f.fangjian@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-loopback-test.c

@@ -875,7 +875,7 @@ static int spi_test_run_iter(struct spi_device *spi,
 		test.transfers[i].len = len;
 		if (test.transfers[i].tx_buf)
 			test.transfers[i].tx_buf += tx_off;
-		if (test.transfers[i].tx_buf)
+		if (test.transfers[i].rx_buf)
 			test.transfers[i].rx_buf += rx_off;
 	}