spi: rzv2h-rspi: set MUST_RX/MUST_TX

spi: rzv2h-rspi: set MUST_RX/MUST_TX

In preparation for implementing DMA support, set MUST_RX and MUST_TX
flags on the controller so that we always receive non-NULL buffers.

The PIO mode already handles this manually by checking if rx_buf/tx_buf
are set on the transfer, and doing a dummy read/write if not.

DMA will not be able to implement this special handling, and although
the SPI controller advertises support for transmit-only or receive-only
transfers via SPCR's register TXMD bitfield, it does not seem to work.

Remove the special handling for PIO and let the SPI controller core
handle it.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Link: https://patch.msgid.link/20251201134229.600817-7-cosmin-gabriel.tanislav.xa@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>