[ALSA] fix HD audio ALC260 mono (un)mute

HDA Codec driver
The ALC260 'Mono Playback Switch' is marked as an output in
patch_realtek.c. It actually does not work unless it is marked as an
input. Go figure... This was tested and confirmed on an HP xw4300.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>