ALSA: usb-audio: Proper fallback at get_term_name()

get_term_name() calls snd_usb_copy_string_desc() for retrieving the
name when a specific ID (name field) is given.  When this returns an
error (zero), however, it simply returns as is.  This will end up in a
fixed name string in the caller side, which often is meaningless.

For giving a bit more useful name string depending on the terminal
type, change the get_term_name() function to go through the fallback
mode.

Signed-off-by: Takashi Iwai <tiwai@suse.de>