net/mlx5: Fix access to a non-supported register

Validate MRTC register is supported before triggering a delayed work
which accesses it.

Fixes: 5a1023de ("net/mlx5: Add periodic update of host time to firmware")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>