net: bcmgenet: Set phydev->dev_flags only for internal PHYs

phydev->dev_flags is entirely dependent on the PHY device driver which
is going to be used, setting the internal GENET PHY revision in those
bits only makes sense when drivers/net/phy/bcm7xxx.c is the PHY driver
being used.

Fixes: 487320c5

 ("net: bcmgenet: communicate integrated PHY revision to PHY driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>