spi_mpc8xxx: Fix uninitialized variable

This patch fixes the following warning:

CC      drivers/spi/spi_mpc8xxx.o
  spi_mpc8xxx.c: In function 'of_mpc8xxx_spi_probe':
  spi_mpc8xxx.c:681: warning: 'ret' may be used uninitialized in this function

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>