kernel/sched: Interpret zero timeslice time correctly

The scheduler API has always allowed setting a zero slice size as a
way to disable timeslicing.  But the workaround introduced for
CONFIG_SWAP_NONATOMIC forgot that convention, and was calling
reset_time_slice() with that zero value (i.e. requesting an immediate
interrupt) in circumstances where z_swap() had been interrupted
nonatomically.

In practice, this never happened.  And if it did, it was a single
spurious no-op interrupt that no one cared about.  Until it did,
anyway...

Now that ticks on nRF devices are at full 32 kHz speed, we can get
into a situation where the rapidly triggering timeslice interrupts are
interrupting z_swap() calls, and the process feeds back on itself and
becomes self-sustaining.

Put that test into the time slice code itself to prevent this kind of
mistake in the future.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>