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>