net/mlx5: fw_tracer, Add support for unrecognized string

In case FW is publishing a string which isn't found in the driver's
string DBs, keep the string as raw data.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>