sysclk-lwla: Avoid warning due to bogus range check.

(lwla_set_clock_source): Checking whether an enum value is greater
than or equal to zero apparently results in a warning with some
compilers.  Assign the enum to an unsigned variable to avoid this,
and return SR_ERR_BUG if the range is exceeded, as this indicates
a bug in the driver code itself.
sysclk-lwla: Remove unused dev_context::next_state.
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.
sysclk-lwla: Bypass divider in external clock mode.

(lwla_setup_acquisition): Set the clock divider bypass
flag to 1 for the external clock modes as well.
(capture_setup): Set the clock divide count to 0 if an
external clock source is selected.
sysclk-lwla: Make use of the probe index again.

(configure_probes): sr_probe::index has been un-deprecated,
so use it instead of relying on the list order.