ALSA: hda - Allow to pass position_fix=0 explicitly

Set the default value of position_fix -1, and allow user passing
position_fix=0 explicitly to set the "auto" position-fix mode.
Otherwise the auto mode may be switched to others like COMBO of
VIACOMBO when the controller prefers it, thus user can't set the auto
mode any longer.

Also updated the documentation appropriately, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -860,8 +860,14 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
 
     [Multiple options for each card instance]
     model	- force the model name
-    position_fix - Fix DMA pointer (0 = auto, 1 = use LPIB, 2 = POSBUF,
-    		   3 = VIACOMBO, 4 = COMBO)
+    position_fix - Fix DMA pointer
+		  -1 = system default: choose appropriate one per controller
+			hardware
+		  0 = auto: falls back to LPIB when POSBUF doesn't work
+		  1 = use LPIB
+		  2 = POSBUF: use position buffer
+		  3 = VIACOMBO: VIA-specific workaround for capture
+		  4 = COMBO: use LPIB for playback, auto for capture stream
     probe_mask  - Bitmask to probe codecs (default = -1, meaning all slots)
     		  When the bit 8 (0x100) is set, the lower 8 bits are used
 		  as the "fixed" codec slots; i.e. the driver probes the