net/mlx5e: Retrieve the switchdev id from the firmware only once

Avoid firmware command execution each time the switchdev HW ID attr get
call is made. We do that by reading the ID (PF NIC MAC) only once at
load time and store it on the representor structure.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>