ALSA: us122l: add snd_us122l_free()

Use it to clean up snd_us122l_card_used[].

Without patch unplugging of an US122L soundcard didn't reset the
corresponding element of snd_us122l_card_used[] to 0.
The (SNDRV_CARDS + 1)th plugging in did not result in creating the soundcard
device anymore.
Index values supplied with the modprobe command line were not used correctly
anymore after the first unplugging of an US122L.

Signed-off-by: Karsten Wiese <fzu@wemgehoertderstaat.de>
Cc: stable@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>