drivers: net: cpsw: use of_platform_depopulate()

Use of_platform_depopulate() in cpsw_remove() instead of
of_device_unregister(), because CSPW child devices will not be
recreated otherwise on next insmod. of_platform_depopulate() is
correct way now as it will ensure that all steps done in
of_platform_populate() are reverted, including cleaning up of
OF_POPULATED flag.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>