net/mlx5: DR, Ignore modify TTL if device doesn't support it

When modifying TTL, packet's csum has to be recalculated.
Due to HW issue in ConnectX-5, csum recalculation for modify TTL
is supported through a work-around that is specifically enabled
by configuration.
If the work-around isn't enabled, ignore the modify TTL action
rather than adding an unsupported action.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>