net: ethernet: ti: cpsw: fix tx vlan priority mapping

The CPDMA_TX_PRIORITY_MAP in real is vlan pcp field priority mapping
register and basically replaces vlan pcp field for tagged packets.
So, set it to be 1:1 mapping. Otherwise, it will cause unexpected
change of egress vlan tagged packets, like prio 2 -> prio 5.

Fixes: e05107e6 ("net: ethernet: ti: cpsw: add multi queue support")
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>