net/mlx4_en: Fixed crash when port type is changed

timecounter_init() was was called only after first potential
timecounter_read().
Moved mlx4_en_init_timestamp() before mlx4_en_init_netdev()

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>