spi: loopback: match configuration of test to description

The test "two tx+rx transfers - alter second" actually modifies
the first not the second transfer, which - in conjunction with
testing the read data - results also in overwriting data read
earlier.
Signed-off-by: NMartin Sperl <kernel@martin.sperl.org>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi-loopback-test.c

@@ -260,7 +260,7 @@ static struct spi_test spi_tests[] = {
 		.fill_option	= FILL_COUNT_8,
 		.iterate_len    = { ITERATE_MAX_LEN },
 		.iterate_tx_align = ITERATE_ALIGN,
-		.iterate_transfer_mask = BIT(0),
+		.iterate_transfer_mask = BIT(1),
 		.transfers		= {
 			{
 				.len = 1,