soundwire: remove bitfield for unique_id, use u8

There is no good reason why the unique_id needs to be stored as 4
bits. The code will work without changes with a u8 since all values
are already filtered while parsing the ACPI tables and Slave devID
registers.

Use u8 representation. This will allow us to encode a
"IGNORE_UNIQUE_ID" value to account for firmware/BIOS creativity.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191022234808.17432-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>