kernel: Do not use sys_clock_ticks_per_sec in _ms_to_ticks()

The value of sys_clock_ticks_per_sec is obtained using
simple integer division with rounding toward zero. As result
using this variable in _ms_to_ticks() introduces some error.

This commit eliminates sys_clock_ticks_per_sec from equation
used in _ms_to_ticks() removing introduced error.

Also, this commit fixes #8895.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>