net: phy: bcm7xxx: Implement EGPHY workaround for 7278

Implement the HW design team recommended workaround in for 7278. Since
the GPHY now returns its revision information in MII_PHYS_ID[23] we need
to check whether the revision provided in flags is 0 or not.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>