drivers/net/at1700.c: fix sparse warning: symbol shadows an earlier one

Impact: Move variable declaration to a more inner scope.

Fix this sparse warning:
  drivers/net/at1700.c:846:21: warning: symbol 'i' shadows an earlier one
  drivers/net/at1700.c:831:13: originally declared here

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