net/sfc: add switch mode device argument

net/sfc: add switch mode device argument

Add the argument that allows user to choose either switchdev or legacy
mode. Legacy mode enables switching by using Ethernet virtual bridging
(EVB) API. In switchdev mode, VF traffic goes via port representor
(if any) on PF, and software virtual switch (for example, Open vSwitch)
steers the traffic.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ivan Malov <ivan.malov@oktetlabs.ru>