ALSA: hda - Move beep attach/detach calls in hda_generic.c

Instead of calling snd_hda_attach_beep_device() and
snd_hda_detach_beep_device() in each codec driver, move them to the
generic parser.  The codec driver just needs to set spec->beep_nid for
activating the digital beep.

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