Add support for rewrite of PCP and DEI value, based on QoS and DP level.

The DCB rewrite table is queried for mappings between priority and
PCP/DEI. The classified DP level is then encoded in the DEI bit, if a
mapping for DEI exists.
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>