net: phy: aquantia: rename aquantia.c to aquantia_main.c

Rename aquantia.c to aquantia_main.c to be prepared for adding new
functionality to separate source code files.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>