drivers/net/niu.c: fix sparse warning: symbol shadows an earlier one

Impact: Move variable declaration to a more inner scope.

Fix this sparse warning:
  drivers/net/niu.c:2399:21: warning: symbol 'err' shadows an earlier one
  drivers/net/niu.c:2287:13: originally declared here

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>