spi: axi-spi-engine: use struct_size() macro

This makes use of the struct_size() macro to calculate the size of the
struct axi_spi_engine when allocating it.

Suggested-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://msgid.link/r/20240304-mainline-axi-spi-engine-small-cleanups-v2-3-5b14ed729a31@baylibre.com


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