tg3: Reorg tg3_napi members

This patch reorders and realigns the tg3_napi members for a ~3-4%
performance improvement on small packet performance tests.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>