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.com


Signed-off-by: Mark Brown <broonie@kernel.org>