net/mlx5: E-Switch, Load/unload VF reps according to event from host PF

When host PF changes the number of VFs, the ECPF esw driver will get
a FW event. It should query the number of VFs enabled by host PF and
update the VF reps accordingly. Note that host PF can't change the
number of VFs dynamically, it has to reset the number of VFs to 0
before changing to a new positive number.

The host event is registered when driver is moving to switchdev mode,
and it's the last step to do in esw_offloads_init. It's unregistered
and the work queue is flushed when driver quits from switchdev mode.
In this way, the host event and devlink command are serialized.

When driver is enabling switchdev mode, pay attention to the following
two facts:
1. Host PF must not have VF initialized as the flow table in ECPF has
   ENCAP enabled as default. Such flow table can't be created with
   existing initialized VFs.
2. ECPF doesn't know how many VFs the host PF will enable, ECPF
   offloads flow steering shall create the flow table/groups based on
   the max number of VFs possibly supported by host PF.

Signed-off-by: Bodong Wang <bodong@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>