net: phy: add a note about refcounting

Recently, a patch has been submitted to "fix" the refcounting for a DT
node in of_mdiobus_link_mdiodev(). This is not a leaked refcount. The
refcount is passed to the new device.

Sadly, coccicheck identifies this location as a leaked refcount, which
means we're likely to keep getting patches to "fix" this. However,
fixing this will cause breakage. Add a comment to state that the lack
of of_node_put() here is intentional.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>