ALSA: ctxfi: Use WARN_ON() instead of BUG_ON()

BUG_ON() is rather useless for debugging as it leads to panic().
Use WARN_ON() and handle the error cases accordingly.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/ctxfi/cthardware.c b/sound/pci/ctxfi/cthardware.c
index 110b8ac..a689f25 100644
--- a/sound/pci/ctxfi/cthardware.c
+++ b/sound/pci/ctxfi/cthardware.c
@@ -69,7 +69,8 @@
 {
 	int i;
 
-	BUG_ON(!field);
+	if (WARN_ON(!field))
+		return 0;
 	/* @field should always be greater than 0 */
 	for (i = 0; !(field & (1 << i)); )
 		i++;
@@ -81,7 +82,8 @@
 {
 	int i;
 
-	BUG_ON(!field);
+	if (WARN_ON(!field))
+		return;
 	/* @field should always be greater than 0 */
 	for (i = 0; !(field & (1 << i)); )
 		i++;