Rename rigol-ds1xx2 driver to rigol-ds.
rigol-ds: Add support for DS2xx2 series.

Based on patch by Mathias Grimmberger <mgri@zaphod.sax.de>.
rigol-ds: fix calls to config_list with NULL sdi/devc.
rigol-ds: brown paper bag, using timebases instead of vdivs.
Fix handling second channel on DS2000 series.

Patch from Mathias Grimmberger.
DS2000 sample memory capture patches from Mathias Grimmberger.
Create & use new sr_usbtmc_dev_inst for Rigol DS driver.