net/mlx5e: Remove timestamp set from netdevice open flow

To avoid configuration override, timestamp set call will
be moved from the netdevice open flow to the init flow.
By this, a close-open procedure will not override the timestamp
configuration.
In addition, the change will rename mlx5e_timestamp_set function
to be mlx5e_timestamp_init.

Fixes: ef9814de ("net/mlx5e: Add HW timestamping (TS) support")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>