[ALSA] soc/wm8731: Fix stereo mixer controls

Disable the simultaneous load feature for the line in and headphone
out volume registers. This allows left and right volume levels to
be controlled separately.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>