fence_cdu: add 8i support (#471)

• Add support for latest Sentry version 8 firmware (Pro1/Pro2 devices)

This has been verfied working on version 7:
Sentry Switched CDU 7.1f
and version 8 devices:
Sentry Switched PDU 8.0i

• Add version function= to differ firmware API

Beside:
Sentry Switched CDU Version 6a
Sentry Switched CDU Version 7.1c

there will be:
Sentry Switched CDU Version 8.0i

which has a slightly different API

• Fix undefined EC_STATUS in error case