Merge branch 'of_mdio-checks'

Sergei Shtylyov says:

====================
of_mdio: use IS_ERR_OR_NULL() and PTR_ERR_OR_ZERO()

   Here's the set of 3 patches against DaveM's 'net-next.git' repo. They deal
with some error checks in the device tree MDIO code...
====================

Signed-off-by: David S. Miller <davem@davemloft.net>