hameg-hmo: re-add 20V/div and 50V/div vertical scales (RTB2K)

hameg-hmo: re-add 20V/div and 50V/div vertical scales (RTB2K)

The absence of the 20V and 50V choices in the driver resulted in failed
probe/scan when the device happened to be configured to these values
(which is more probable with disabled channels). Re-introduce vdivs[]
items which unconditionally were removed in commit 4f3cb1eaf727. Use
them for RTB2K models, stick with at most 10V/div for all other models
to remain compatible with the previous implementation.

[ use 'git diff -U25' to see the models and verify the change ]