amixer: Make "dB" case-insensitive in set commands

We don't have to be necessarily too strict about case-sensitivity of
"dB" suffix used in set commands.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
amixer: Parse the value more strictly

So far amixer allows some unexpected suffix and assumes as a raw
absolute value without returning an error.  This is rather dangerous,
e.g. user might not notice that a completely wrong value was set when
the command line included a typo.

This patch makes the parser a bit more strict: it doesn't allow any
longer invalid suffixes, instead either returns an error or skips the
invalid value, depending on the operation mode.

Signed-off-by: Takashi Iwai <tiwai@suse.de>