IB/mlx5: Fix port speed query for representors

IB/mlx5: Fix port speed query for representors

When querying speed information for a representor in switchdev mode,
the code previously used the first device in the eswitch, which may not
match the device that actually owns the representor. In setups such as
multi-port eswitch or LAG, this led to incorrect port attributes being
reported.

Fix this by retrieving the correct core device from the representor's
eswitch before querying its port attributes.

Fixes: 27f9e0ccb6da ("net/mlx5: Lag, Add single RDMA device in multiport mode")
Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/20260115-port-speed-query-fix-v2-1-3bde6a3c78e7@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>