spi: gpio: fix const issue in spi_to_spi_gpio()

spi: gpio: fix const issue in spi_to_spi_gpio()

While the struct spi_device * passed into spi_to_spi_gpio() is a const
one, the struct spi_bitbang * that is retrieved from the controller
field in the spi_device is NOT a const pointer, as it is coming from the
spi_controller_get_devdata() call, and then passed to container_of()
which would strip off the const attribute for no good reason (i.e. if a
const pointer is passed to container_of() it still is const coming out).

Fix this all up by properly declaring the struct spi_bitbang * as not
const.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/2025052225-scallion-ritzy-dbbd@gregkh
Signed-off-by: Mark Brown <broonie@kernel.org>