net: phy: mscc: factorize code for LEDs mode

LEDs modes are set the same way, except they are offset by 4 times the
index of the LED.

Let's factorize all the code so that it's easier to add support for the
4 LEDs of the VSC8584 PHY.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>