lib: os: clock: fix for CID 529870

Fix for CID 529870, where Coverity found an issue where `timespec.tv_sec`
is never greater than `UINT64_MAX / NSEC_PER_SEC` (18446744073).

This is naturally true when `time_t` is only 32-bit, which is actually
never the case for any Zephyr platform aside from `native_sim/native/32`.

When `time_t` is a signed 64-bit value, at some point in the future, but
maybe not in our lifetimes, `timespec.tv_sec` could exceed 18446744073,
since `INT64_MAX > UINT64_MAX / NSEC_PER_SEC`.

We should not see coverity issues errors like this in the future, once we
have a consistent `time_t` representation across all Zephyr platforms.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>