net: stmmac: Fix race between stmmac_drv_probe and stmmac_open

There is currently a small window during which the network device registered by
stmmac can be made visible, yet all resources, including and clock and MDIO bus
have not had a chance to be set up, this can lead to the following error to
occur:

[  473.919358] stmmaceth 0000:01:00.0 (unnamed net_device) (uninitialized):
                stmmac_dvr_probe: warning: cannot get CSR clock
[  473.919382] stmmaceth 0000:01:00.0: no reset control found
[  473.919412] stmmac - user ID: 0x10, Synopsys ID: 0x42
[  473.919429] stmmaceth 0000:01:00.0: DMA HW capability register supported
[  473.919436] stmmaceth 0000:01:00.0: RX Checksum Offload Engine supported
[  473.919443] stmmaceth 0000:01:00.0: TX Checksum insertion supported
[  473.919451] stmmaceth 0000:01:00.0 (unnamed net_device) (uninitialized):
                Enable RX Mitigation via HW Watchdog Timer
[  473.921395] libphy: PHY stmmac-1:00 not found
[  473.921417] stmmaceth 0000:01:00.0 eth0: Could not attach to PHY
[  473.921427] stmmaceth 0000:01:00.0 eth0: stmmac_open: Cannot attach to
                PHY (error: -19)
[  473.959710] libphy: stmmac: probed
[  473.959724] stmmaceth 0000:01:00.0 eth0: PHY ID 01410cc2 at 0 IRQ POLL
                (stmmac-1:00) active
[  473.959728] stmmaceth 0000:01:00.0 eth0: PHY ID 01410cc2 at 1 IRQ POLL
                (stmmac-1:01)
[  473.959731] stmmaceth 0000:01:00.0 eth0: PHY ID 01410cc2 at 2 IRQ POLL
                (stmmac-1:02)
[  473.959734] stmmaceth 0000:01:00.0 eth0: PHY ID 01410cc2 at 3 IRQ POLL
                (stmmac-1:03)

Fix this by making sure that register_netdev() is the last thing being done,
which guarantees that the clock and the MDIO bus are available.

Fixes: 4bfcbd7abce2 ("stmmac: Move the mdio_register/_unregister in probe/remove")
Reported-by: Kweh, Hock Leong <hock.leong.kweh@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>