spi: make class structs const

The two instances of struct class are only used here in functions that
take const pointers and so can too be made constant.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://patch.msgid.link/20241018122437.64275-1-brgl@bgdev.pl


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