net: netcp: ethss: fix up incorrect use of list api

The code seems to assume a null is returned when the list is empty
from first_sec_slave() to break the loop which is incorrect. Fix the
code by using list_empty().

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>