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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-loopback-test.c

···
 		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;
 	}