r6040: check for absent PHY

Some devices have two R6040 MACs but the second one
is not wired to any PHY, therefore the interface is
just unusable. Warn the user about that and prevent
device from registering.

Tested-by: bifferos <bifferos@yahoo.co.uk>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>