ppc4xx: Fix compilation warning for PIP405

This patch fixes a compilation warning for the PIP405 board. It moves the
#ifndef CONFIG_CS8952_PHY define a little so that the warning doesn't
occur anymore. I am a little unsure if this #ifdef is at the correct
place now or if it could be removed completely. This needs to get
tested on the PIP405 board.

Signed-off-by: Stefan Roese <sr@denx.de>