net: bcmgenet: Poll internal PHY for GENETv5

On GENETv5, there is a hardware issue which prevents the GENET hardware
from generating a link UP interrupt when the link is operating at
10Mbits/sec. Since we do not have any way to configure the link
detection logic, fallback to polling in that case.

Fixes: 42138085 ("net: bcmgenet: add support for the GENETv5 hardware")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>