lorawan: clock_sync: assert instead of error code

Assert that the size provided to `clock_sync_serialize_device_time` is
sufficient, instead of returning an error code. The condition is already
enforced by the calling code, and returning an error code is incorrect
when the return code is unconditionally being used to increment the
buffer offset by callers.

Fixes #84720.

Signed-off-by: Jordan Yates <jordan@embeint.com>