net: phy: bcm7xxx: Make MII_BCM7XX_64CLK_MDIO naming consistent

The driver is BCM7xxx, we were missing an additional X in the constant naming,
fix that to be consistent.

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