net: phy: mscc: factorize function for getting LED mode from DT

Microsemi PHYs support different LED modes depending on the variant, so
let's factorize the code so we just have to give the supported modes
while the logic behind getting the mode remains identical.

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