bridge: mrp: Rename br_mrp_port_open to br_mrp_ring_port_open

This patch renames the function br_mrp_port_open to
br_mrp_ring_port_open. In this way is more clear that a ring port lost
the continuity because there will be also a br_mrp_in_port_open.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>