FWU: Don't report failure if everything is up to date [REVPI-1336]

Don't report failure, when firmware is up to date

A return code of 0 is used to signal, that everythin is up to date. Therefore FWU_update should not issue an error message for this case (only for ret < 0).

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Don't try to update base

A RevPi base module has no firmware, which can be updated via piControl. Therefore the base device (address 0) needs to be skipped in FWU_update.