spi: mpc512x-psc: fix compiler warning about uninitialized variable

This fixes several warnings like:

	drivers/spi/spi-mpc512x-psc.c: In function 'mpc512x_psc_spi_prep_xfer_hw':
	arch/powerpc/include/asm/io.h:163:2: warning: '__ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

introduced in commit 8bf96098 for some build configurations.

Fixes: 8bf96098 ("spi: mpc512x-psc: add support for Freescale MPC5125")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>