net/mlx5: Move some PPS logic into helper functions

Some of PPS logic (timestamp calculations) fits only internal timer
timestamp mode. Move these logics into helper functions. Later in the
patchset cyc2time HW translation mode will expose its own PPS timestamp
calculations.

With this change, main flow will only hold calling PPS logic based on run
time mode.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>