pthread: cond: fix pthread_cond_wait always returning ETIMEDOUT

It was noted that `pthread_cond_wait()` would always return
ETIMEDOUT, even when successful (and no timeout should ever
occur with `K_FOREVER`).

The z_sched_wake() / z_sched_wake_all() / z_sched_wait() API
are used here with a swap return value of 0 to indicate
success.

Fixes #41284

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>