drivers: net: ethernet: cpsw: consider number of slaves in interation

Make cpsw_add_default_vlan() look at the actual number of slaves for its
iteration, so boards with less than 2 slaves don't ooops at boot.

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