net/mlx5: Allocate rate limit table when rate is configured

A device supports 128 rate limiters. A static table allocation consumes
8KB of memory even when rate is not configured.

Instead, allocate the table when at least one rate is configured.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>