driver: net: ldpaa: Update priv->phydev after free()

Even after memory free of phydev, priv is still pointing to the
obsolete address.
So update priv->phydev as NULL after memory free.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Signed-off-by: Ashish Kumar <Ashish.Kumar@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>