ALSA: hda: Abort capability probe at invalid register read

The loop in snd_hdac_bus_parse_capabilities() may go to nirvana when
it hits an invalid register value read:

 BUG: unable to handle kernel paging request at ffffad5dc41f3fff
 IP: pci_azx_readl+0x5/0x10 [snd_hda_intel]
 Call Trace:
  snd_hdac_bus_parse_capabilities+0x3c/0x1f0 [snd_hda_core]
  azx_probe_continue+0x7d5/0x940 [snd_hda_intel]
  .....

This happened on a new Intel machine, and we need to check the value
and abort the loop accordingly.

[Note: the fixes tag below indicates only the commit where this patch
 can be applied; the original problem was introduced even before that
 commit]

Fixes: 6720b38 ("ALSA: hda - move bus_parse_capabilities to core")
Cc: <stable@vger.kernel.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/hdac_controller.c

@@ -284,6 +284,11 @@ int snd_hdac_bus_parse_capabilities(struct hdac_bus *bus)
 		dev_dbg(bus->dev, "HDA capability ID: 0x%x\n",
 			(cur_cap & AZX_CAP_HDR_ID_MASK) >> AZX_CAP_HDR_ID_OFF);
 
+		if (cur_cap == -1) {
+			dev_dbg(bus->dev, "Invalid capability reg read\n");
+			break;
+		}
+
 		switch ((cur_cap & AZX_CAP_HDR_ID_MASK) >> AZX_CAP_HDR_ID_OFF) {
 		case AZX_ML_CAP_ID:
 			dev_dbg(bus->dev, "Found ML capability\n");