[ALSA] sound: es1968.c fox shadowed variable warning

id is used when initializing the mixer elements, use elem_id here
instead.
sound/pci/es1968.c:1963:25: warning: symbol 'id' shadows an earlier one
sound/pci/es1968.c:129:13: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>