spi: Don't use flexible array in struct spi_message definition

The struct spi_message can be embedded into another structures.
With that the flexible array might be problematic as sparse
complains about it, although there is no real issue in the code
because when the message is embedded it doesn't use flexible array
member. That memeber is a private to spi_message_alloc() API, so
move it to that API in a form of an inherited data type.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Fixes: 75e308ff ("spi: Use struct_size() helper"))
Closes: https://lore.kernel.org/r/20231009-onshore-underage-c58415adfd92-mkl@pengutronix.de


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20231010163100.89734-1-andriy.shevchenko@linux.intel.com


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