Tinyalsa: Show the correct error message for negative value

Currently, if the user tries to set a mixer with a
negative value, tinymix displays the following error:
"Error: only enum types can be set with strings".

This patch purpose is to inform the user that
the negative values are not allowed with the error
message "Error: invalid value for index"

Category: aosp improvement
Domain: AudioComms-Tinyalsa
Origin: Internal
Upstream-Candidate: Yes

Change-Id: I8d55d02512764c71d19ec50158224d4e978b9967
Signed-off-by: Baptiste Robert <baptistex.robert@intel.com>