spi: rzv2h-rspi: set TX FIFO threshold to 0

In PIO mode we send data word-by-word, and wait for the received data
to be available after each sent word, making no use of the TX interrupt.

In DMA mode, we need to set the RX and TX FIFO thresholds to 0, as
described in the User Manual.

In preparation for implementing DMA support, set TX FIFO threshold to 0,
as RX FIFO threshold is already 0.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Link: https://patch.msgid.link/20251201134229.600817-8-cosmin-gabriel.tanislav.xa@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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