spi: Constify spi_get_ctldata()'s spi parameter

The "spi" parameter of spi_get_ctldata() can be const.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/8960e07adaad8d92d2c3aa045af9ee3c5d2130a8.1678704562.git.geert+renesas@glider.be


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