When snd_hdac_refresh_widget_sysfs() is called before the first
hda_widget_sysfs_init(), the next call overrides and eventually
fails.  This results in unexpected Oops, something like:
  BUG: unable to handle kernel NULL pointer dereference at 00000000000000c8
  IP: [<ffffffff8180e2a3>] hdmi_chmap_ctl_info+0x23/0x40

The fix is to add a check of the existing sysfs tree.  Also, for more
safety, this patch adds the checks of device_is_registered() in
snd-hdac_refresh_wdiget_sysfs(), too.

Fixes: fa4f18b4 ('ALSA: hda - Refresh widgets sysfs at probing Haswell+ HDMI codecs')
Bugizlla: https://bugzilla.kernel.org/show_bug.cgi?id=103431Reported-by: Andreas Reis <andreas.reis@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>