spi: ppc4xx: Drop write-only variable

[ Upstream commit b3aa619a ]

Since commit 24778be2 ("spi: convert drivers to use
bits_per_word_mask") the bits_per_word variable is only written to. The
check that was there before isn't needed any more as the spi core
ensures that only 8 bit transfers are used, so the variable can go away
together with all assignments to it.

Fixes: 24778be2

 ("spi: convert drivers to use bits_per_word_mask")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20240210164006.208149-8-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>