audio/transport: change volume to 8bit

audio/transport: change volume to 8bit

The valid range of volume is 0 - 127, yet it is stored in 16bit
data type. This patch modifies it so we use 8bit data type to
store volume instead. Furthermore we also use signed type, so
negative values can be used to indicate invalid volume.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>