sysclk-lwla: Utility functions clean-up and semantic fixes.

sysclk-lwla: Utility functions clean-up and semantic fixes.

(lwla_send_bitstream): Unref the mapped file earlier in order
to simplify the error handling.
(lwla_receive_reply): Do not treat a reply buffer length of
zero as silent no-op.  That logic was left over from an earlier
iteration, before the distinction between reply buffer size and
expected read length was introduced.