[POWERPC] mpc5200-fec: Fix possible NULL dereference in mdio driver

If the reg property is missing from the phy node (unlikely, but possible),
then the kernel will oops with a NULL pointer dereference.  This fixes
it by checking the pointer first.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>