spi_mpc8xxx: Factor out SPI mode change steps into a call

We'll add more steps soon, so get rid of the duplication.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>