ALSA: fireface: fix memory leak in ff400_switch_fetching_mode()

An allocated memory forgets to be released.

Fixes: 76fdb3a9

 ('ALSA: fireface: add support for Fireface 400')
Cc: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>