net: ethernet: ti/cpsw: do not crash on single-MAC machines during resume

During resume, use for_each_slave to walk the slaves of the cpsw, and
soft-reset each of them. This prevents oopses if there is only one
slave configured.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>