soc: intel_adsp: ipc: don't call k_sem_init() multiple times

k_sem_init() is called for every IPC message sent in
intel_adsp_ipc_send_message(). This has not had any side-effects
in upstream configurations, but has been linked to a failing
run of test_obj_tracking_sanity test case in downstream Zephyr
use.

Replace k_sem_init() with k_sem_reset() as this is more appropriate
API to reset the semaphore count, and ensure deterministic
behaviour in case a thread is waiting on the semaphore at time
of reset.

Suggested-by: Peter Mitsis <peter.mitsis@intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>