[ALSA] sound: ac97_pcm.c fix shadowed variable warning

err is always assigned before it is used, no need to declare another
inside the if statement.
sound/pci/ac97/ac97_pcm.c:577:7: warning: symbol 'err' shadows an earlier one
sound/pci/ac97/ac97_pcm.c:572:6: originally declared here

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