tg3: Fix sparse warnings.

drivers/net/ethernet/broadcom/tg3.c:8121:8: warning: symbol 'i' shadows an earlier one
drivers/net/ethernet/broadcom/tg3.c:8003:6: originally declared here
drivers/net/ethernet/broadcom/tg3.c:785:5: warning: symbol 'tg3_ape_scratchpad_read' was not declared. Should it be static?
drivers/net/ethernet/broadcom/tg3.c:7781:19: warning: Using plain integer as NULL pointer
drivers/net/ethernet/broadcom/tg3.c:10231:31: error: bad constant expression

Reported-by: Fengguang Wu <fenguang.wu@intel.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>