net: bcmgenet: fix RGMII_MODE_EN bit

RGMII_MODE_EN bit was defined to 0, while it is actually 6. It was not
much of a problem on older designs where this was a no-op, and the RGMII
data-path would always be enabled, but newer GENET controllers need to
explicitely enable their RGMII data-pad using this bit.

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