[ALSA] sound: ca0106_mixer.c fix shadowed variable warnings

Change the variable err to _err within the ADD_CTLS macro to avoid
shadowing the local variable.

sound/pci/ca0106/ca0106_mixer.c:710:2: warning: symbol 'err' shadows an earlier one
sound/pci/ca0106/ca0106_mixer.c:663:6: originally declared here
sound/pci/ca0106/ca0106_mixer.c:712:3: warning: symbol 'err' shadows an earlier one
sound/pci/ca0106/ca0106_mixer.c:663:6: originally declared here
sound/pci/ca0106/ca0106_mixer.c:721:3: warning: symbol 'err' shadows an earlier one
sound/pci/ca0106/ca0106_mixer.c:663:6: originally declared here

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