spi: ppc4xx: Fix fallout from rename in struct spi_bitbang

spi: ppc4xx: Fix fallout from rename in struct spi_bitbang

I failed to adapt this driver because it's not enabled in a powerpc
allmodconfig build and also wasn't hit by my grep expertise. Fix
accordingly.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402100815.XQXw9XCF-lkp@intel.com/
Fixes: 2259233110d9 ("spi: bitbang: Follow renaming of SPI "master" to "controller"")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://msgid.link/r/20240210164006.208149-7-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>