ALSA: fireface: fix for state to fetch PCM frames

According to my memo at hand and saved records, writing 0x00000001 to
SND_FF_REG_FETCH_PCM_FRAMES disables fetching PCM frames in corresponding
channel, however current implement uses reversed logic. This results in
muted volume in device side during playback.

This commit corrects the bug.

Cc: <stable@vger.kernel.org> # v4.12+
Fixes: 76fdb3a9e1 ('ALSA: fireface: add support for Fireface 400')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/fireface/ff-protocol-ff400.c

@@ -86,7 +86,7 @@ static int ff400_switch_fetching_mode(struct snd_ff *ff, bool enable)
 	if (reg == NULL)
 		return -ENOMEM;
 
-	if (enable) {
+	if (!enable) {
 		/*
 		 * Each quadlet is corresponding to data channels in a data
 		 * blocks in reverse order. Precisely, quadlets for available