drivers/net/igb: fix sparse warning: symbol shadows an earlier one

Impact: Move variable declaration as far inner as possible.

Fix this sparse warning:
  drivers/net/igb/igb_main.c:1895:21: warning: symbol 'j' shadows an earlier one
  drivers/net/igb/igb_main.c:1855:16: originally declared here

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