solos: Add 'Firmware' attribute for Traverse overall firmware version

The existing 'FirmwareVersion' attribute only covers the DSP firmware as
provided by Conexant; not the overall version of the device firmware. We
do want to be able to see the full version number too.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>