net/mlx5: Check DTOR entry value is not zero

The Default Timeout Register (DTOR) provides timeout values to driver
for flows that are device dependent. Zero value for DTOR entry is not
valid and should not be used. In case of reading zero value from DTOR,
the driver should use the hard coded SW default value instead.

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