net: phylink: avoid mac_config calls

Avoid calling mac_config() when using split PCS, and the interface
remains the same.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>