spi: Constify spi_get_drvdata()'s spi parameter

The "spi" parameter of spi_get_drvdata() can be const.
dev_get_drvdata() has been taking a const pointer since commit
7d1d8999 ("i2c: Constify i2c_get_clientdata's parameter").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/0f1700ade27a8f3935d04480ff7bef8a887331eb.1678704562.git.geert+renesas@glider.be


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