net/mlx5: E-Switch, Protect from invalid memory access in offload fdb table

The esw offloads structures share a union with the legacy mode structs.
Reset the offloads struct to zero in init to protect from null
assumptions made by the legacy mode code.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>