alsactl: correctly restore dB values of controls with changed range

alsactl: correctly restore dB values of controls with changed range

When the range of a control has changed between driver versions, it is a
good idea to restore the same dB value of the control.  However,
computing the dB value by interpolating betweem the min/max dB values
duplicates alsa-lib's TLV functions and does not even work for controls
with a linear dB range.

A simple conversion to dB and back can be done if we add the dB value(s)
to the saved state.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>