net: bcmgenet: Fix RGMII_MODE_EN value for GENET v1/2/3

The RGMII_MODE_EN bit value was 0 for GENET versions 1 through 3, and
became 6 for GENET v4 and above, account for that difference.

Fixes: aa09677c

 ("net: bcmgenet: add MDIO routines")
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>