Implement handlers for the devlink commands to get and set the SRIOV
E-Switch mode.

When turning to the switchdev/offloads mode, we disable the e-switch
and enable it again in the new mode, create the NIC offloads table
and create VF reps.

When turning to legacy mode, we remove the VF reps and the offloads
table, and re-initiate the e-switch in it's legacy mode.

The actual creation/removal of the VF reps is done in downstream patches.
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>