spi: loopback-test: write rx pattern also when running without tx_buf

Currently the rx_buf does not get set with the
SPI_TEST_PATTERN_UNWRITTEN when tx_buf == NULL in the transfer.

Reorder code so that it gets done also under this specific condition.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>