drivers: net: cpsw: remove child devices while driver detach

remove all the child devices from the system to make sure that re-insert of
cpsw module doesn't fail on child device populated by of_platform_populate().

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