tg3: Move producer ring struct to tg3_napi

Now that each NAPI instance has its own producer ring, it no longer
makes sense to keep the producer ring structure external.  This patch
migrates the producer ring struct to tg3_napi and pivots the code to the
new implementation.

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