ALSA: ctxfi: cthw20k2: fix mask on conf to allow 4 bits

Currently the mask operation on variable conf is just 3 bits so
the switch statement case value of 8 is unreachable dead code.
The function daio_mgr_dao_init can be passed a 4 bit value,
function dao_rsc_init calls it with conf set to:

     conf = (desc->msr & 0x7) | (desc->passthru << 3);

so clearly when desc->passthru is set to 1 then conf can be
at least 8.

Fix this by changing the mask to 0xf.

Fixes: 8cc72361 ("ALSA: SB X-Fi driver merge")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210227001527.1077484-1-colin.king@canonical.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/ctxfi/cthw20k2.c

@@ -991,7 +991,7 @@ static int daio_mgr_dao_init(void *blk, unsigned int idx, unsigned int conf)
 
 	if (idx < 4) {
 		/* S/PDIF output */
-		switch ((conf & 0x7)) {
+		switch ((conf & 0xf)) {
 		case 1:
 			set_field(&ctl->txctl[idx], ATXCTL_NUC, 0);
 			break;