spi: stm32: move splitting transfers to optimize_message

Since splitting transfers was moved to spi_optimize_message() in the
core SPI code, we now need to use the optimize_message callback in the
STM32 SPI driver to ensure that the operation is only performed once
when spi_optimize_message() is used by peripheral drivers explicitly.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://msgid.link/r/20240219-mainline-spi-precook-message-v2-3-4a762c6701b9@baylibre.com


Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>