tools: ynl-gen: fix enum index in _decode_enum(..)

tools: ynl-gen: fix enum index in _decode_enum(..)

Remove wrong index adjustment, which is leftover from adding
support for sparse enums.
enum.entries_by_val() function shall not subtract the start-value, as
it is indexed with real enum value.

Fixes: c311aaa74ca1 ("tools: ynl: fix enum-as-flags in the generic CLI")
Signed-off-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230725101642.267248-2-arkadiusz.kubalewski@intel.com
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>