sfc: create VEB vswitch and vport above default firmware setup

Adds functions to allocate and free vswitches and vports; vadaptors
are automatically allocated and freed when TX/RX queues are
initialised and finalised.  This vswitching structure is only created
if the firmware supports it, so a check that full-featured firmware
is running is performed first.

If the MC resets, the vswitching infrastructure will need to be
recreated, so mark the "must_probe_vswitching" flag when an MC reboot
is detected.

Don't try to create a vswitch if vf-count=0

This allocation of vswitches and vports does not currently support
configuring VLAN tags, but that can be added in a future change.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>