net/designware: call phy_connect_dev() to properly setup phylib device

This sets up the linkage from the phydev back to the ethernet device. This
symptom of not doing this which I noticed was:
    <NULL> Waiting for PHY auto negotiation to complete....
rather than:
    dwmac.1c50000 Waiting for PHY auto negotiation to complete....

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>