drivers: net:cpsw: fix probe_dt when only slave 1 is pinned out

when slave 0 has no phy and slave 1 connected to phy, driver probe will
fail as there is no phy id present for slave 0 device tree, so continuing
even though no phy-id found, also moving mac-id read later to ensure
mac-id is read from device tree even when phy-id entry in not found.

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