drivers: timer: nrf_grtc_timer: Fix for random system hangs

d599e2b6 removed early return from sys_clock_timeout_handler if
current counter value is less than cc_val. It seems that this return is
needed as after the removal some stress tests which were using system
timers heavily started to hang.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>