ALSA: scarlett2: Minor refactor MSD mode check

Create local variable for storing private data pointer in
snd_scarlett2_controls_create(). It's currently only used for checking
msd_switch, but it will be used again.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/ab50939dca0fdc5fa3493fc8eee3a2fdefffa62d.1703612638.git.g@b4.vu

sound/usb/mixer_scarlett2.c

@@ -5898,6 +5898,7 @@ static int snd_scarlett2_controls_create(
 	struct usb_mixer_interface *mixer,
 	const struct scarlett2_device_entry *entry)
 {
+	struct scarlett2_data *private;
 	int err;
 
 	/* Initialise private data */
@@ -5905,6 +5906,8 @@ static int snd_scarlett2_controls_create(
 	if (err < 0)
 		return err;
 
+	private = mixer->private_data;
+
 	/* Send proprietary USB initialisation sequence */
 	err = scarlett2_usb_init(mixer);
 	if (err < 0)
@@ -5931,7 +5934,7 @@ static int snd_scarlett2_controls_create(
 		return err;
 
 	/* If MSD mode is enabled, don't create any other controls */
-	if (((struct scarlett2_data *)mixer->private_data)->msd_switch)
+	if (private->msd_switch)
 		return 0;
 
 	/* Create the analogue output controls */