[ALSA] Interpret negative index as bitmask of permissible indexes

Documentation,ALSA Core
Currently arguments of the 'index' option from 0 through SNDRV_CARDS-1
force a module to take the specified index. Index -1 makes the module
take the first available index. This patch extends this convention so
that a negative index value is interpreted as a bitmask of the
permitted indexes. Special cases:

    -1 0xffffffff 0 and up
    -2 0xfffffffe 1 and up
    -4 0xfffffffc 2 and up
    ...

The patch includes also corrections of ALSA-Configuration.txt document.
Signed-off-by: Thomas Hood <jdthood@yahoo.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/init.c

@@ -84,16 +84,13 @@ snd_card_t *snd_card_new(int idx, const char *xid,
 	write_lock(&snd_card_rwlock);
 	if (idx < 0) {
 		int idx2;
-		for (idx2 = 0; idx2 < snd_ecards_limit; idx2++)
-			if (!(snd_cards_lock & (1 << idx2))) {
+		for (idx2 = 0; idx2 < SNDRV_CARDS; idx2++)
+			if (~snd_cards_lock & idx & 1<<idx2) {
 				idx = idx2;
+				if (idx >= snd_ecards_limit)
+					snd_ecards_limit = idx + 1;
 				break;
 			}
-		if (idx < 0 && snd_ecards_limit < SNDRV_CARDS)
-			/* for dynamically additional devices like hotplug:
-			 * increment the limit if still free slot exists.
-			 */
-			idx = snd_ecards_limit++;
 	} else if (idx < snd_ecards_limit) {
 		if (snd_cards_lock & (1 << idx))
 			err = -ENODEV;	/* invalid */