net/mlx5: Fix return value from tracer initialization

Check return value of mlx5_fw_tracer_start(), set error path and fix
return value of mlx5_fw_tracer_init() accordingly.

Fixes: c71ad41c ("net/mlx5: FW tracer, events handling")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>