drivers/net/cxgb3: fix sparse warning: symbol shadows an earlier one

Impact: Move variable declaration as close to usage as possible.

Fix this sparse warning:
  drivers/net/cxgb3/cxgb3_main.c:1586:21: warning: symbol 'cap' shadows an earlier one

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>