[nrf fromtree] tests: drivers: spi_loopback: Fix NULL rx buff testcase

Fixes setup of TX buffer, which shouldn't be NULL in the RX NULL
tescase.

Signed-off-by: Raffael Rostagno <[email protected]>
(cherry picked from commit d8358b2)

Author: Raffael Rostagno

tests/drivers/spi/spi_loopback/src/spi.c

@@ -559,7 +559,7 @@ ZTEST(spi_loopback, test_spi_null_rx_buf_set)
 {
 	struct spi_dt_spec *spec = loopback_specs[spec_idx];
 	const struct spi_buf_set tx = spi_loopback_setup_xfer(tx_bufs_pool, 1,
-							      NULL, BUF_SIZE);
+							      buffer_tx, BUF_SIZE);
 
 	spi_loopback_transceive(spec, &tx, NULL);
 }