phylib: don't return NULL from get_phy_device()

Arnd Bergmann asked that get_phy_device() returns either NULL or the error
value,  not both on error.  Do as he said, return ERR_PTR(-ENODEV) instead
of NULL when the PHY ID registers read as  all ones.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>