rigol-ds: Handle digital channels correctly for MSO2000A series.

rigol-ds: Handle digital channels correctly for MSO2000A series.

The handling of the digital channels for this series is somewhere
between that of the DS1000D series (PROTOCOL_V2) and the MSO1000Z
(PROTOCOL_V4).

The :LA command set is similar to that of V4, but the LA data has to be
requested with :WAV:SOUR:LA and arrives in interleaved form like V2.

None of these changes should affect other models. They only affect the
case of PROTOCOL_V3 with digital channels, which occurs only for the
MSO2000A series.