spi: spi-mt65xx: Fix NULL pointer access in interrupt handler

[ Upstream commit a20ad45008a7c82f1184dc6dee280096009ece55 ]

The TX buffer in spi_transfer can be a NULL pointer, so the interrupt
handler may end up writing to the invalid memory and cause crashes.

Add a check to trans->tx_buf before using it.

Fixes: 1ce24864bf ("spi: mediatek: Only do dma for 4-byte aligned buffers")
Signed-off-by: Fei Shao <fshao@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://msgid.link/r/20240321070942.1587146-2-fshao@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/spi/spi-mt65xx.c

@@ -659,6 +659,7 @@ static irqreturn_t mtk_spi_interrupt(int irq, void *dev_id)
 		mdata->xfer_len = min(MTK_SPI_MAX_FIFO_SIZE, len);
 		mtk_spi_setup_packet(master);
 
+		if (trans->tx_buf) {
 			cnt = mdata->xfer_len / 4;
 			iowrite32_rep(mdata->base + SPI_TX_DATA_REG,
 					trans->tx_buf + mdata->num_xfered, cnt);
@@ -671,6 +672,7 @@ static irqreturn_t mtk_spi_interrupt(int irq, void *dev_id)
 					remainder);
 				writel(reg_val, mdata->base + SPI_TX_DATA_REG);
 			}
+		}
 
 		mtk_spi_enable_transfer(master);