spi: mediatek: fix fifo rx mode

spi: mediatek: fix fifo rx mode

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: a568231f4632 ("spi: mediatek: Add spi bus for Mediatek MT8173")
Signed-off-by: Peter Hess <[email protected]>
Signed-off-by: Frank Wunderlich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>