net/mlx5e: Use read lock for eswitch get callbacks

In commit 367dfa12 ("net/mlx5: Remove devl_unlock from
mlx5_eswtich_mode_callback_enter") all functions were converted
to use write lock without relation to their actual purpose.

Change the devlink eswitch getters to use read and not write locks.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>