cxgb4: add main VI to mirror VI config replication

When mirror VI is enabled, replicate various VI config params
enabled on main VI to mirror VI. These include replicating MTU,
promiscuous mode, all-multicast mode, and enabled netdev Rx
feature offloads.

v3:
- Replace mirror VI refcount_t with normal u32 variable.
- Add back calling cxgb4_port_mirror_start() in cxgb_open(), which
  was there in v1, but got missed in v2 during refactoring.

v2:
- Simplify the replication code by refactoring t4_set_rxmode()
  to handle mirror VI, instead of duplicating the t4_set_rxmode()
  calls in multiple places.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>