kernel/timeout: Fix elapsed logic
kernel/timeout: Fix elapsed logic

When fetching the next timeout to expire, the value is relative to the
last announced tick, so you subtract the timer-provided elapsed time
to get the true delta from "now".  When adding a new timeout, you
*have* a value relative to now, so you compute the delta vs. the last
announced tick by adding the elapsed() time.  Duh.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,punitvara/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr