ALSA: x86: Don't bail out from PCM ops when disconnected

Currently the driver returns -ENODEV when the monitor is disconnected.
But PA alsa module doesn't like this and it starts playing Juliet,
kills itself as if it were a fatal tragedy.

Since we protect the whole read/write at disconnection, just allow the
PCM accesses even during disconnection.

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