spi: more tx_buf/rx_buf removal

Merge series from David Lechner <dlechner@baylibre.com>:

I found a couple more controller drivers that were checking if the
tx_buf and rx_buf fields in the spi_transfer structure were set by a
peripheral driver that I missed in [1]. These checks can be removed
as well.

[1]: https://lore.kernel.org/linux-spi/20240325-spi-remove-is_dma_mapped-v2-1-d08d62b61f1c@baylibre.com/