kernel: Fix timeout issue with SYSTEM_CLOCK_SLOPPY_IDLE

We can't simply use CLAMP to set the next timeout because
when CONFIG_SYSTEM_CLOCK_SLOPPY_IDLE is set, MAX_WAIT is
a negative number and then CLAMP will be called with
the higher boundary lower the lower boundary.

Fixes #41422

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>