net: phy: Properly define genphy_c45_driver

Avoid the W=1 warning that symbol 'genphy_c45_driver' was not
declared. Should it be static?

Declare it on the phy header file.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>