ucm: Add command 'get _file' to get the config file name of the opened card

After opening a card, this command can show the name of the actually
loaded configuration file, either matches the card name or card long name.
So developers can check if there is a device-sepcific configuration file
available for a given card.

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>