spi: spi-fsl-dspi: Fix issue with uninitialized dma_slave_config

Depending on the DMA driver being used, the struct dma_slave_config may
need to be initialized to zero for the unused data.

For example, we have three DMA drivers using src_port_window_size and
dst_port_window_size. If these are left uninitialized, it can cause DMA
failures.

For spi-fsl-dspi, this is probably not currently an issue but is still
good to fix though.

Fixes: 90ba3703 ("spi: spi-fsl-dspi: Add DMA support for Vybrid")
Cc: Sanchayan Maity <maitysanchayan@gmail.com>
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Cc: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Cc: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210810081727.19491-1-tony@atomide.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-fsl-dspi.c

@@ -530,6 +530,7 @@ static int dspi_request_dma(struct fsl_dspi *dspi, phys_addr_t phy_addr)
 		goto err_rx_dma_buf;
 	}
 
+	memset(&cfg, 0, sizeof(cfg));
 	cfg.src_addr = phy_addr + SPI_POPR;
 	cfg.dst_addr = phy_addr + SPI_PUSHR;
 	cfg.src_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;