drivers: net: ethernet: cpsw: get slave VLAN id from slave node instead of cpsw node

Dual EMAC slave VLAN id must be got from slave node instead of cpsw node as
VLAN id for each slave will be different.

Reported-by: Mark Jackson <mpfj-list@mimc.co.uk>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>