drivers: net: cpsw: fix cpsw dp parse when num slaves as 1

drivers: net: cpsw: fix cpsw dp parse when num slaves as 1

On some boards number of slaves can be 1 when only one port
ethernet is pinned out. So do not break when slave_index and
num slaves check fails, instead continue to parse the next
child.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>