net: phy: properly report internal PHYs through sysfs

Internal PHYs may not have a valid PHY interface defined, which will
show up in sysfs as "". Add an explicit check of internal PHYs to report
their interface correctly.

Fixes: 3d055d8d

 ("net: phy: expose PHY device interface mode")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>