spi: loopback-test: correct mismatched test description and configuration

The test "two tx-transfers - alter first" actually alters the second
not the first transfer.  Similarly the test "two tx-transfers - alter
second" actually alters the first not the second transfer.

The mismatches for the two symmetrical tests cancel each other's
mistakes.  But it's better to fix the mismatches to avoid confusion.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>