spi: imx: drop useless member speed_hz from driver data struct

The driver data's member variable just caches the transfer's speed_hz
member. All users of the former now have access directly to the latter.
So fix them to use the uncached value and remove the cache.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>