spi: stm32: use callbacks for read_rx and write_tx

The STM32F7 will require different read and write routines, so make
these functions into configurable callbacks.

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>
Link: https://lore.kernel.org/r/20231102193722.3042245-3-ben.wolsieffer@hefring.com


Signed-off-by: Mark Brown <broonie@kernel.org>