tinymix: fix setting enum str started with digits

For those value strings which are started with digits, they must be set
as enum instead of index value.

Test: with mixer control values started with digits like '48KHz'
Change-Id: I1c70f5613a48d020d3248b71c1e4384f83e33d25
Signed-off-by: HW Lee <hwlee@google.com>