sound/oss: fix sparse warning: symbol shadows an earlier one

Impact: Move variable to a more inner scope.

Fix this sparse warning:
  sound/oss/sequencer.c:235:29: warning: symbol 'err' shadows an earlier one
  sound/oss/sequencer.c:215:13: originally declared here

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>