In FIFO mode were two problems:
- RX mode was never handled and
- in this case the tx_buf pointer was NULL and caused an exception

fix this by handling RX mode in mtk_spi_fifo_transfer

Fixes: a568231f ("spi: mediatek: Add spi bus for Mediatek MT8173")
Signed-off-by: Peter Hess <peter.hess@ph-home.de>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Link: https://lore.kernel.org/r/20210706121609.680534-1-linux@fw-web.deSigned-off-by: Mark Brown <broonie@kernel.org>