hantek-6xxx: Ignore requests to set Hantek 6022BE coupling.

hantek-6xxx: Ignore requests to set Hantek 6022BE coupling.

Here's a patch to "Ignore requests to set coupling for the Hantek 6022BE",
this clears the LIBUSB errors for me.

Also in the patch:

 - There is a crash because config_list() can be called with sdi == NULL.

   This can be reproduced by doing:
     "sigrok-cli.exe --driver hantek-6xxx --show"

 - There seems to be a very unsafe loop in config_set() when setting COUPLING;
   the coupling vector is assumed to be zero terminated, but is not declared
   as such.

   Note: The same issue is present also for other hardware, at least for
   hantek-dso/api.c. The patch is only for hantek-6xxx though.