net: phy: at803x: use phy_modify_mmd()

Convert at803x_clk_out_config() to use phy_modify_mmd().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1kyc72-0008Pq-1x@rmk-PC.armlinux.org.uk


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