ALSA: core api: define offsets for TLV items

Currently, there are no pre-defined accessors for the elements
in topology TLV data. In the absence of such offsets, the
tlv data will have to be decoded using hardwired offset
numbers 0-N depending on the type of TLV. This patch defines
accessor offsets for the type, length, min and mute/step items
in TLV data for DB_SCALE type tlv's. These will be used by drivers to
decode the TLV data while loading topology thereby improving
code readability. The type and len offsets are common for all TLV
types. The min and step/mute offsets are specific to DB_SCALE tlv type.

Signed-off-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>