drivers: net: cpsw: increment reference count on fixed-link PHY node

When a fixed-link sub-node exists in a slave node, the slave node
is also the PHY node. Since this is a separate use of the slave node,
of_node_get() should be used to increment the reference count.

Fixes: 1f71e8c9 ("drivers: net: cpsw: Add support for fixed-link PHY")
Signed-off-by: David Rivshin <drivshin@allworx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>