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

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: 1ce24864 ("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.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mt65xx.c

@@ -788,6 +788,7 @@ 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(host);
 
+		if (trans->tx_buf) {
 			cnt = mdata->xfer_len / 4;
 			iowrite32_rep(mdata->base + SPI_TX_DATA_REG,
 					trans->tx_buf + mdata->num_xfered, cnt);
@@ -800,6 +801,7 @@ 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(host);