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: a568231f46322 ("spi: mediatek: Add spi bus for Mediatek MT8173")
Signed-off-by: Benjamin Gaignard <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-mt65xx.c

···
 	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,