amixer - Fix selection of enum items

amixer - Fix selection of enum items

The item names with channel specifier cannot be chosen correctly
via amixer sset.  For example, 'Front Mic' is handled as 'Mic' since
the channel specificier 'Front' is omitted.

The idea of channel specifier is basically non-sense for enum items,
and this also makes the code very messy.

The patch fixes the behavior of enum items and gives the code clean-up
for sset command.