spi: spi-zynq-qspi: Fix stack violation bug

When the number of bytes for the op is greater than one, the read could
run off the end of the function stack and cause a crash.

This patch restores the behaviour of safely reading out of the original
opcode location.
Signed-off-by: Karen Dombroski <karen.dombroski@marsbioimaging.com>
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Link: https://lore.kernel.org/r/20210429053802.17650-3-amit.kumar-mahapatra@xilinx.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-zynq-qspi.c

@@ -528,18 +528,17 @@ static int zynq_qspi_exec_mem_op(struct spi_mem *mem,
 	struct zynq_qspi *xqspi = spi_controller_get_devdata(mem->spi->master);
 	int err = 0, i;
 	u8 *tmpbuf;
-	u8 opcode = op->cmd.opcode;
 
 	dev_dbg(xqspi->dev, "cmd:%#x mode:%d.%d.%d.%d\n",
-		opcode, op->cmd.buswidth, op->addr.buswidth,
+		op->cmd.opcode, op->cmd.buswidth, op->addr.buswidth,
 		op->dummy.buswidth, op->data.buswidth);
 
 	zynq_qspi_chipselect(mem->spi, true);
 	zynq_qspi_config_op(xqspi, mem->spi);
 
-	if (op->cmd.nbytes) {
+	if (op->cmd.opcode) {
 		reinit_completion(&xqspi->data_completion);
-		xqspi->txbuf = &opcode;
+		xqspi->txbuf = (u8 *)&op->cmd.opcode;
 		xqspi->rxbuf = NULL;
 		xqspi->tx_bytes = op->cmd.nbytes;
 		xqspi->rx_bytes = op->cmd.nbytes;