spi: spi-gpio: Remove spi->controller_data comment

The conversion from GPIO numbers to GPIO descriptors removed the use of
spi->controller_data, but forgot to update a comment referring to it.

Fixes: 9b00bc7b ("spi: spi-gpio: Rewrite to use GPIO descriptors")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>