net/mlx5: Avoid printing health buffer when firmware is unavailable

Use firmware version field as an indication to health buffer's sanity.
When firmware version is 0xFFFFFFFF, deduce that firmware is unavailable
and avoid printing the health buffer to dmesg as it doesn't provide
debug info.

Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>