net/fec_mpc52xx: Don't dereference phy_device if it is NULL

The FEC Ethernet device isn't always attached to a phy.  Be careful
not to dereference phy_device if it is NULL.  Also eliminates an
unnecessary extra function from the ioctl path.

Reported-by: Henk Stegeman <henk.stegeman@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>