spi: Make several public functions private to spi.c

spi: Make several public functions private to spi.c

All these functions have no callers apart from drivers/spi/spi.c. So
drop their declarations in include/linux/spi/spi.h and don't export
them.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211007121415.2401638-5-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>