kernel: fix k_sleep in no multi-threading mode

Fix k_sleep implementation for no multi-threading mode.

Absolute value of timeout expiration was fed to the k_busy_wait()
function instead of delta value. That caused bug like incrementing of
sleep time in geometric progression (while actual function argument is
constant) during program running.

Signed-off-by: Mikhail Kushnerov <m.kushnerov@yadro.com>