net: phy: aquantia: move MMD_VEND define to header

Move MMD_VEND define to header to clean things up and in preparation for
firmware loading support that require some define placed in
aquantia_main.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>