kernel: sched: fix legacy timeout calculation in z_tick_sleep

Ticks should be assigned directly to timeout value in case of
CONFIG_LEGACY_TIMEOUT_API=y, just as they were before referenced patch.

Fixes: 7a815d5d ("kernel: sched: Use k_ticks_t in z_tick_sleep")
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>