ALSA: hda - Move beep helper functions to hda_beep.c

Move snd_hda_mixer_amp_switch_put_beep() to hda_beep.c as a clean up
to remove one more ifdef.

Also add the corresponding get callback to return consistently the
digital beep state independently from the mixer amp value.

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