sfc: use a dynamic m-port for representor RX and set it promisc

Representors do not want to be subject to the PF's Ethernet address
 filters, since traffic from VFs will typically have a destination
 either elsewhere on the link segment or on an overlay network.
So, create a dynamic m-port with promiscuous and all-multicast
 filters, and set it as the egress port of representor default rules.
 Since the m-port is an alias of the calling PF's own m-port, traffic
 will still be delivered to the PF's RXQs, but it will be subject to
 the VNRX filter rules installed on the dynamic m-port (specified by
 the v-port ID field of the filter spec).

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>