posix: mutex: return ETIMEDOUT from pthread_mutex_timedlock()

The normative spec for `pthread_mutex_timedlock()` says that
it should return `ETIMEDOUT` when a timeout occurs. However,
currently it returns `EAGAIN`, which reflects what is returned
by `k_mutex_lock()`.

Inspect and update the return value to account for this slight
difference.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>