[ALSA] fix behaviour of ac97_enum_mixer elements

AC97 Codec
ac97_enum mixer elements (e.g., 'Capture Source') did not work because
of wrong bitmask calculation in snd_ac97_get_enum_double() and
snd_ac97_put_enum_double().

https://bugtrack.alsa-project.org/alsa-bug/view.php?id=1072



Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>