net: phy: make genphy_c45_write_eee_adv() static

genphy_c45_write_eee_adv() isn't used outside phy-c45.c,
so make it static.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/d23bd784-44e6-4a15-af3a-b37379156521@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>