nano_timer: Make nano_timer_init call _nano_timeout_init

With the introduction of _nano_timeout_init it prefered to call it
to initialize the _nano_timeout fields properly.

Change-Id: I83e9c63f9bb2903c508264d1199d2c687c330ec8
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>