ALSA: x86: Fix racy access to chmap

The access to chmap can be racy against the hotplug process, where it
recreates the chmap on the fly.  For protecting against it, a mutex is
introduced in this patch.  It's also used for protecting the change /
reference of eld and state fields, too.

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