spi: mediatek: Avoid NULL pointer crash in interrupt

In some case, like after a transfer timeout, master->cur_msg pointer
is NULL which led to a kernel crash when trying to use master->cur_msg->spi.
mtk_spi_can_dma(), pointed by master->can_dma, doesn't use this parameter
avoid the problem by setting NULL as second parameter.

Fixes: a568231f ("spi: mediatek: Add spi bus for Mediatek MT8173")
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Link: https://lore.kernel.org/r/20220131141708.888710-1-benjamin.gaignard@collabora.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mt65xx.c

@@ -624,7 +624,7 @@ static irqreturn_t mtk_spi_interrupt(int irq, void *dev_id)
 	else
 		mdata->state = MTK_SPI_IDLE;
 
-	if (!master->can_dma(master, master->cur_msg->spi, trans)) {
+	if (!master->can_dma(master, NULL, trans)) {
 		if (trans->rx_buf) {
 			cnt = mdata->xfer_len / 4;
 			ioread32_rep(mdata->base + SPI_RX_DATA_REG,