staging/line6: Fix sparse warning 'Using plain integer as NULL pointer'

This patch fixes the warning generated by sparse: "Using plain integer
as NULL pointer" by replacing the offending 0s with NULL.
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/line6/capture.c

@@ -147,7 +147,7 @@ void line6_capture_copy(struct snd_line6_pcm *line6pcm, char *fbuf, int fsize)
 	const int bytes_per_frame = line6pcm->properties->bytes_per_frame;
 	int frames = fsize / bytes_per_frame;
 
-	if (runtime == 0)
+	if (runtime == NULL)
 		return;
 
 	if (line6pcm->pos_in_done + frames > runtime->buffer_size) {

drivers/staging/line6/midi.c

@@ -127,7 +127,7 @@ static int send_midi_async(struct usb_line6 *line6, unsigned char *data,
 
 	urb = usb_alloc_urb(0, GFP_ATOMIC);
 
-	if (urb == 0) {
+	if (urb == NULL) {
 		dev_err(line6->ifcdev, "Out of memory\n");
 		return -ENOMEM;
 	}
@@ -137,7 +137,7 @@ static int send_midi_async(struct usb_line6 *line6, unsigned char *data,
 
 	transfer_buffer = kmalloc(length, GFP_ATOMIC);
 
-	if (transfer_buffer == 0) {
+	if (transfer_buffer == NULL) {
 		usb_free_urb(urb);
 		dev_err(line6->ifcdev, "Out of memory\n");
 		return -ENOMEM;

drivers/staging/line6/playback.c

@@ -246,7 +246,7 @@ static int submit_audio_out_urb(struct snd_line6_pcm *line6pcm)
 
 	change_volume(urb_out, line6pcm->volume_playback, bytes_per_frame);
 
-	if (line6pcm->prev_fbuf != 0) {
+	if (line6pcm->prev_fbuf != NULL) {
 #ifdef CONFIG_LINE6_USB_IMPULSE_RESPONSE
 		if (line6pcm->flags & MASK_PCM_IMPULSE) {
 			create_impulse_test_signal(line6pcm, urb_out,