[ALSA] es18xx: Enable wavetable input from ESS chips

This patch enables wavetable chips ES689/ES69X connected to
ESS ES18xx chips. The wavetable chip uses FM DAC if the clock signal
from the wavetable is active.
It has no effect if there is no ESS wavetable chip present.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>