hameg-hmo: Add support for SR_CONF_LOGIC_THRESHOLD/_CUSTOM.

hameg-hmo: Add support for SR_CONF_LOGIC_THRESHOLD/_CUSTOM.

Update the Hameg/Rohde&Schwarz HMO driver (hameg-hmo) so that it
is possible to configure the logic threshold for digital signals.

The user can get or set the logic threshold configuration using
the channel group POD0 (and/or POD1 where available), for example:

sigrok-cli --driver hameg-hmo --get logic_threshold -g POD0
sigrok-cli --driver hameg-hmo --config logic_threshold=TTL --set -g POD0

sigrok-cli --driver hameg-hmo --get logic_threshold_custom -g POD0
sigrok-cli --driver hameg-hmo --config logic_threshold_custom=0.7 --set -g POD0