ALSA: hda - Fix beep_mode option value

The beep_mode option value was wrongly defined: it must be 0 = off and
1 = on.

Also, evaluate the beep_mode value at snd_hda_attach_beep_device()
properly so that no device is created when beep_mode=0 is given.

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