ppc4xx: Fix compilation breakage in miiphy.c

Patch 8ef583a0 [miiphy: convert to linux/mii.h] introduced a small
problem in the ppc4xx miiphy.c version. This patch fixes this problem.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>