net: ethernet: ti: cpsw: fix secondary-emac probe error path

Make sure to deregister the primary device in case the secondary emac
fails to probe.

kernel BUG at /home/johan/work/omicron/src/linux/net/core/dev.c:7743!
...
[<c05b3dec>] (free_netdev) from [<c04fe6c0>] (cpsw_probe+0x9cc/0xe50)
[<c04fe6c0>] (cpsw_probe) from [<c047b28c>] (platform_drv_probe+0x5c/0xc0)

Fixes: d9ba8f9e ("driver: net: ethernet: cpsw: dual emac interface
implementation")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>