net: phy: bcm7xxx: introduce helper macro

All 28nm Gigabit PHYs supported by the driver have the same
callbacks, the only differences being the 32-bits OUI and the name. Use
a macro to factor this, making it easier in the future to add new
entries.

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