net: phy: micrel: Try default PHY ofnode first

The phydev structure has a PHY OF node pointer in it, use that OF node
first when looking up PHY OF node properties, since that is likely the
correct PHY OF node pointer. If the pointer is not valid, which is the
case e.g. on legacy DTs, fall back to parsing MAC ethernet-phy subnode.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>